Hey y'all- relatively new poster here.
 
I recently purchased 2 1080's and have them set up in 2xSLI. Normally, the primary card does fine and temp throttles when it reaches 83C under load. The secondary card has no thermal issues. The primary card on rare occasion seems to disable its own temp throttle and just run itself full power until the temps reach in excess of 95C and the fans come on full-blast (at which point I quit out of whatever game I'm playing). I've used both SpeedFan and MSI Afterburner to try to monitor temps and confirm the readings so I know they're accurate. I've also tried to use MSI Afterburner to manually throttle back the card, but inevitably it still climbs well above the 83C thermal limit. I have noticed that rebooting the machine does seem to clear up the issue so I suspect this may be an obscure driver bug.
 
I have closely examined case cooling and found no issues there. I know the card does manage to run just fine most times with a constant temp around 82-83C when under load. 
 
Has anyone else run into an issue like this? Any ideas?
 
Thanks!

Yeah, I saw that issue. No black screens here or system crashes- the card simply goes well above the thermal limit of 83C. Nothing else of note really occurs beyond that. When the fans spin up to 100%, it's very brief- lasts only a few seconds, then back to normal. Reboot the machine and it seems to clear up.

Someone else had a post about this same issue where they were exceeding the temp limit but I haven't found that specific thread yet.

Is there an extra slot spacing between the two cards, or are they right next to each other?

Assuming you are venting out the top and or top-back of your case, the top one always gets hotter...as it's pulling in the hot exhaust air from the bottom one to cool itself.  This is why I don't like ACX style coolers for SLI.  The blower style coolers work better as the hot air is exhausted from the case instead of being dumped into the case.
 
I would try the following (in this order):
- 1. Increase your case airflow (give it more intake air)...if you are running your intake fans at less than 100% RPM, crank them all the way up and see what improved airflow does.
- 2. Increase your case exhaust air flow (suck out more of the hot air).
- 3. Do both increased intake and exhaust.
- 4. Swap the order of the cards and see if the "hot" card moves from top to bottom, or if the new top card is now the new hot card.
- 5. Try placing an extra fan by the bottom card...forcing air to the back of the case (i.e. forcing the hot exhaust air away from the top card.)
- 6. Try doing the same thing for the top card.

Thanks for the tips y'all! 
 
I've examined case cooling thoroughly already in a myriad of configurations and adding/removing fans as well as adding intake/exhaust to the side of the case directly adjacent to the cards themselves. Nothing has had any real effect there. 
 
Swapping the cards is something that hasn't occurred so I will do that today. I'll also do a driver wipe/reinstall too while I'm at it. I'll also check the heatsink / screws as well. 
 
I'm amazed your cards run that cool considering what all I've seen with mine so far. I'm using an old Antec case that was a limited edition with a Guild Wars 2 logo plate- tower case. It's got 2 fans on top (exhaust), 2 fans top-rear (also exhaust), and 3 fans bottom-front (intake). The PSU is not isolated, but it exhausts bottom-rear. I have negative pressure inside the case. Ambient temp inside the house is ~78F.
 
I take it that the Thermaltake Core X9 would be a much more optimal setup for SLI?

Also - AHowes said he was running Founders Edition cards...AKA "blower style" cooler.  These types of coolers exhaust the hot air from cooling the graphics card outside of the case.
 
ACX style coolers (aka non-blower cooler) exhaust all of the GPU hot air into the case...hence increasing the temperature of the surrounding stuff.  You need plenty of airflow to get the heat out.
 
Think of it this way, with an ACX style cooler, you are putting at least 500 W of heat (200 W per GPU, and 100 W CPU/MB) into a space smaller than 1 cubic meter.  That's 5 100 W light bulbs in a medium size shipping box...things will get warm.

I do unfortunately have the ACX coolers on the FTW's. 
 
I swapped the cards with each other and noted that now, both cards are running around the same temps rather than one lagging behind a good 10-15C. At this point, I'm getting 82C with the primary and around 80C on the secondary (originally the primary) while under load. This seems to indicate to me that there may be a problem with the heat sink on the original primary/now secondary. Still, the temps are running pretty high still (granted I haven't encountered the holy-crap-let's-unleash-this-to-95C so far). While idling, both cards run about 40C (much lower than the 60ish from before).
 
I also decided- mostly for giggles- to point a box fan at the open case and put it on max to see what it'd do to the temps. With a box fan, the temps drop to around 76C for both cards under load. Granted, this thing is making a whole lot of noise, but it is keeping the cards cool at least. Running case fans on maximum without the box fan yields about a 1C drop in temp- not very much. 
 
Also, a case is worth a thousand words, I figure. Hard drives are located above the 3 red fans in the lower right, so they don't interfere with airflow. I am honestly coming to the conclusion I need a better case solution for airflow as it stands. This case has started to feel cramped in the later years of its life... 
 

 
Also on the table is potentially punching a hole in the wall and running cabling for the PC into another room that has somewhat better ventilation; have the 4K TV and all that in one room, PC in the other. Also carries the advantage of eliminating fan noise entirely... 

That's a very busy case!  A couple of observations:
 
1. Your case is on the floor.  Is it by chance under a desk too?
- I originally had my case under a desk.  The hot exhaust air was getting trapped under the desk, and causing the intake fans to bring in air hotter than the room ambient.  When I put it on top of a the desk, the temperatures dropped across by board by 5 C for me.
 
2. Your CPU cooler
- I can't tell if it's blowing hot air down or pulling it up.  If it's pulling it up, it could be pulling GPU hot air up and increasing your CPU temperatures.  If it's pushing the hot air down, this is increasing the temperature of the backside of the top SLI card.
- Try turning the CPU cooler 90 degrees such that is takes in are from the right, and exhausts to the left.
 
3. Your Power Supply
- It looks like this is drawing air from inside of your case versus drawing air from outside.
- This explains some why the bottom card is cooler, as the PSU intake fan is drawing heat off the bottom card.
- This is normally not good as it causes the power supply to be much hotter than it should be
 
4. It looks like the top 2, and back 2 fans are exhaust.  Are the front fans set as intake?
- I hope your intake fans have a decent static pressure (over 1 mmH20) to fight all of the intake restrictions you have.
 
5. You have a lot of drives and cables blocking your front intake fans (if they are intake).
- You should try to clean this up some.  (It looks like you have 2 DVD type drives...do you need 2?  hehe)
- If you can remove one of your DVD type drives, you can get an "icy dock" 5.25 adapter.  This will hold 1 3.5" drive and 2 2.5" drives.  This will remove some drives from your intake fan clutter.  (I run two of these in my second server system.)
- If that doesn't work for you, you can get an external 5.25" enclose that connects to your PC by USB 3.0.  My case doesn't have a 5.25 external bay, and I use this for my BluRay drive.  Only turn it on when I need it (the environment will love you!)
 
6. You can try getting a small fan and mounting to the back grill above your graphics cards.
- You can make the measurements and see what sort of fan you can get in there.
- I've done this before...you can usually get at least 2 screw holes to line up with the mesh
 
7. You should clean up your cable wiring!
 
If you are looking at a new case, check out the Corsair Air 540!

I'll just go down the list!
 
1) On the floor, yes- but not under the desk. I did this a long time ago but learned the error of my ways pretty quickly simply by the sweat of sitting under a desk with a hot computer.
 
2) The CPU cooler is blowing upwards. CPU temps haven't been a huge concern, but that's not a bad idea. I've been running a i5 2500K OC'ed at 4.3 for the better part of the last few years and the guy is rock-solid.
 
3) Good point. I guess that's the value of a case where the power supply is isolated from the rest of the system, yeah?
 
4) Yes, the front 3 are intake. I'm not sure on the specifics of the fans in the case though- goes to show how old the thing is now. I can't even find the case specs online anymore. 
 
5) Also good ideas! I'll purge the old DvD drive and just leave the BlueRay. At some point I had a reason for them both but I don't remember what it is anymore. Ahem. I'll move the hard drives up north after. 
 
6) I'll look into this as well. 
 
7) The picture isn't terribly flattering, but it looks better in person I think. The SATA cables are my biggest problem right now- everything else (with the exception of the 4 pin CPU cable) is routed behind the mobo. The 4 pin won't reach any other way, sadly, so I've tried to get it out of the way as best I could. 
 
Is there a blower cooler for the EVGA 1080 ACX FTW? Or am I asking the wrong question here?
 
Have to start looking at cases, sooner or later!

1 slot spacing is just fine for water cooling.. pretty much 1 slot spacing is the new norm.

Ek makes blocks that use a seperate bridge that will connect both blocks. I know.. I've bought the blocks and bridge 3 times already and returned it between all 4 sets of 1080s I've tried and ended up returning. :O

There's a newer corsair case coming out called the air 740 that looks sweet.. also evga's new cases are coming out shortly I believe that also are bad ass.

The thermal take core x9 is nice as well for water cooling ans air flow but I've had it now for like 2 years and I can't see anything from the side window but my graphics card! Love to go back to a traditional vertical mb case but have not seen many I would run to.

Kind of a final wrap up here before I open a support ticket...
 
I've extensively reworked the cooling in the case- CPU cooler rotated 90 degrees, cleared away a lot of errant cabling and removed the DVD drive and relocated the hard disks away from the fans. It has definitely had an effect of lowering system temperatures, though under full load the cards are still getting into the ~83C range (which I'm not surprised about honestly- this is the normal temp limit and the throttle point). I even ended up turning off the top exhaust fans which resulted in a net cooling gain for the graphics cards (serious **** moment- they must have been pulling cool air up before it could get to the cards), but the primary card just ups itself to take the headroom on up to 83C. The secondary is a good 5-10C cooler now in any case. None of this is necessarily bad- it's not really a big issue.
 
However, I ran into this bug again where the GPU stopped temp throttling itself and began to climb up to the 90's so I had to shut down my game and reboot the machine. Sure enough, after the reboot, temps under load returned to the normal 83C. This IS the big issue here because the card shouldn't be doing this regardless of the cooling situation. At this point I'm going to reinstall drivers for the GPU and hope that clears it up. If it manifests again it's time to open a ticket. 
 
Thanks very much to everyone who replied to this thread with advice! 

It shouldn't stop throttling and head for the 90's. 83-85c would be normal after a while with ACX cards in SLI through. The cards basically choke each other out, as they are both dumping a lot heat into the case. 

Tonight I tested my new evga 1080 classifieds that have the same heatsink as the ftw card. It didn't do too bad. Each card by itself did 50-54c max. 72f in the room.
With sli it did get worse in the upper 50s low 60s which was shocking actually. Note the fan curve was aggressive at 80% at 40c and 100% at 45c hehe.. but still way better then the zotac amp edition with simular double fan large heatsink.

The one card I believe needs to go back though which sucks. It runs default speeds (and everything else in evga's POC at default) with voltage limit "1". If i try a small overclock on it to 2000mhz from the default of 1975mhz it boots to I get 1's in everything for tempt,voltage and power even though everything is fine and low!

Also the card at default runs at 1.064v! Seriously high for defaults am I wrong??

Nuts.