So about 6 weeks ago I put a Hydro Copper on a RTX 2080 XC ULTRA.  Everything has been working great.  Two days ago my frame rates started dropping thru the floor - pulling up Precision X1 I see my card refuses to run faster than 300mhz.  I do the usual updated drivers etc - no joy.  I put the stock fan back on - and the card is back to normal - runs great!  I put the hydro copper back on just to confirm that was the original issue - sure enough no faster than 300mhz.  I'm fairly certain the hydro copper is on correctly.  Any Ideas?

sounds like a thermal issue, card is downclocking to save itself from destruction lol
 
using the right screws in the correct holes??
have enough tension on the screws? 
good thermal paste spread?
thermal pads installed on the right pieces? 

That's been reported before and caused by the cooler contacting the PCB/card components where it shouldn't be, but I can't tell you exactly where at the moment..That's why it's OK when you put the stock cooler back on.
 
Weird that it was OK for about 6 weeks though..Not sayin' that's your issue, but you can check on that.
 
RMA'ing the Hydro Copper might be in your future.

Might wanna Contact EVGA Customer Service about it.

ATGSteakHouse
Finally Fixed - kougar, your post (and you too sparetime) had me recheck the location of the thermal pads - that was it!  One of the two small pads pictured shifted slightly - moved maybe a 1/4"!  That was all it took for my card to lock at 300mhz.  For anyone else who is locked at 300 MHz - make sure your pads are in the correct location - it doesn't take much. (What's odd is it worked for 6 weeks before I see this problem?)  I had to reassemble my loop 5 or 6 times to figure this out - probably 10 hours total - water cooling can be a pain!

Awesome, glad its working again for you now. 

I am glad to hear you got it sorted out! Thermal pads do tend to move around a lot, it's definitely something to double check on water & air coolers alike. 
 
Ever since I had a choke pop on my Titan Black Hydro I've become uber paranoid about PWM temps, and is why I am a strong proponent of iCX enabled cards. iCX may be excessive, but it gives me peace of mind to know there's no block flow or thermal pad issues. As for watercooling being complicated... I admit that is exactly why I now take the lazy approach and just source Hydrocopper models from EVGA so I don't have to worry about block compatibility or anything.  Once you start trying to match blocks with GPUs, it gets crazy fast. EVGA alone had something around 5 PCB variants of the old 1080 Ti cards.

ATGSteakHouse
Finally Fixed - kougar, your post (and you too sparetime) had me recheck the location of the thermal pads - that was it!  One of the two small pads pictured shifted slightly - moved maybe a 1/4"!  That was all it took for my card to lock at 300mhz.  For anyone else who is locked at 300 MHz - make sure your pads are in the correct location - it doesn't take much. (What's odd is it worked for 6 weeks before I see this problem?)  I had to reassemble my loop 5 or 6 times to figure this out - probably 10 hours total - water cooling can be a pain!

That's great news and yes, troubleshooting with a custom loop can be a pain but the gains from custom cooling supersede the cons.
 
The pads after usage, warmth, etc. I would think, move a tad so my guess, it moved enough to where there was some type of bad contact but all is well.