So, I just upgraded to the EVGA 1080 Ti FTW3 from my old 970, using a 8 to 6+2 (x2) split cable from my CX750M PSU (it only has 1 GPU 8pin slot).
When I test with VRMark Orange I score about 7528, but when I try blue my PC restarts anywhere from instantly-20 seconds in. I can have it finish by lowering my Power Target in Precision/Afterburner to about 75-80%, but it still crashes in the 3DMark benchmarks.
From what I've read is that it sounds like the PSU isn't big enough, but the 1080Ti should only draw 2-300, and PCPP says my build should only be 442W (Precision says 650W, idk what to believe).
I've ran DDU to remove my old drivers, and reinstalled the driver for my 1080. I've checked all of my fans and they run fine (GPU, Case, PSU, and Noctua fans). Temps from what I can see are normally 40-50C.
I've tried with, and without Precision and afterburner.
Any Ideas?
 

CPU: Intel - Core i5-4690K 3.5GHz Quad-Core Processor
CPU Cooler: Noctua - NH-D15 82.5 CFM CPU Cooler
Motherboard: MSI - Z97-GAMING 5 ATX LGA1150 Motherboard
Memory: G.Skill - Ripjaws X Series 16GB (2 x 8GB) DDR3-1600 Memory
Storage: Samsung - 850 EVO-Series 500GB 2.5" Solid State Drive
Video Card: EVGA - GeForce GTX 1080 Ti 11GB FTW3 GAMING iCX Video Card
Case: Thermaltake - Overseer RX-I ATX Full Tower Case
Power Supply: Corsair - CXM 750W 80+ Bronze Certified Semi-Modular ATX Power Supply
Optical Drive: Samsung - SH-224FB/RSMS DVD/CD Writer
Operating System: Microsoft - Windows 10 Home OEM 64-bit

Try the other psu terminal & pci-e cables to see if it helps stop the restarting under load.

The only other 8 pin socket is for a 4+4 cpu, unless I'm misunderstanding. I will try the other cable and see if it works any better.
Unfortunately this is my only desktop so I can't test it in other machines.

Well, I tried the other cable and left the power target at 100%, it finished the VRMark blue, and most of the 3DMark, but it restarted during the combined test. Could it be that my PSU can't support both the GPU and CPU when they're both being pushed?

Took a picture, am I just misreading it? I didn't notice the "&" before, but I was under the impression that you couldn't switch them around.

Well out of desperation I tried using both cables, but it crashed at the same point unfortunately. Any recommendations for a PSU? I've been seeing suggestions of 650-750, but if my 750 (though its a 2 year old bronze 750) doesn't cut it, I'm not sure where to start.

Ok, I'll try that. Thanks for the help.

Hey, I noticed that the 850W Titanium is the same price after the rebate. Is there any reason not to go with it instead of the 750?
 
Edit: Just noticed that it doesn't qualify for free shipping, so it's not exactly the same price, and it'll take longer for shipping. My new monitor should be here on Wed, so would it be worth it to upgrade, or is that extra 100W just overkill for now?

Just a FYI - the unit seems to barely be a 650 Watt  in  wolfs clothing - http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story6&reid=416
 
& it is only a bronze rating ,so with the years & heat , entirely possible it can't keep up anymore  -BUT- remember Corsair has great warranty service .  Yours carries a 5 year warranty , so I'd look into getting it replaced for free.  
  
( keep it for a backup )

Not sure if you got your answer but it's because your PSU has only one 6+2pci-e port and one 4+4 eps port.  You will need a PSU that has two 8pin pci-e or two 6pin+2 pci-e connections.  I am actually surprised you didn't fry your hardware trying to start it with the wrong connectors!

I have a very competent PSU but was getting reboots after upgrading to the Titan Xp. Adding another 8pin from the PSU the reboots stopped. Bite the bullet and get a new power supply. "850W Titanium" should serve you well with my emphasis on the 'Titanium' part.