If you are overclocking, notch timings back a bit. Mine started acting up again today once it passed 83 degrees in my room for the first time in months, and things were getting pretty hot in the case. Weather has been out of whack the last couple weeks, and we had a run of hot weather the last 3 days. After I dropped the FSB to 433MHz it seemed to stabilize--was able to run DRG for about 2 hours of Fates with no problems. Would like to still run over 4GHz on that 3GHz CPU (and breaking 1100 on the RAM was nice too)... but, stability matters more then the couple of frames I picked up running things that high.

Might want to double check your heatsink mounts too, just to be sure they are secured well since you re-applied thermal past to them. Low capacity on a PSU is always something to suspect as well. The last couple generations of graphics cards really need an 80+ Certified (or SLI Certified) power supply. If it isn't guaranteed with one of those (or similar) labels, then you may not be getting what it says it is capable of--might only be getting 60% of it's rating under heavy load or when it starts to heat up. I've also seen the fans on a PSU go bad shortly after it was taken out the box and installed. May want to verify all your fans are turning properly as well--especially on the video card (may need to unlock the overclocking feature of the driver and manually set a higher fan speed--common problem with ATI cards).