Are you certain the GPU is overheating? If it's a factory overclock, I suggest acquiring the factories "tuning" utility that can alter voltages, clockspeeds, etc. and UNDERclocking the GPU enough Mhz to get it to the reference speed. If the card works when underclocked, call the mfr for an RMA. I have zero issues doing the same with EVGA. I've had stellar support from them and they keep getting my dollars.
Otherwise, inspect your case closely to be certain the intake and exhaust fans are actually working on your case. Crack it open and clean it with a vacuum and/or take it outside and blow it clean with some compressed air.
One way to help test whether it's truly an overheating problem would be to take the side of the case off and point a desk-type or similar fan directly into the case and see if you can still recreate the results.
Good luck and let us know how it works out!