I have been through several graphics cards (3 Radeon 5870s and a 6950) in the last six months and they all have been acting defective. The last three have been producing a lot of artifacts, both in games and outside of games, even after the manufacturer assured me that they had passed their diagnostic tests, so I've been running a GPU memory test (MemtestCL) and it frequently finds memory errors. I have a well-rated XFX power supply that should be providing more than enough power for these cards, and I'm running them all at stock clocks. I've tried testing with different motherboards, memory, and power supplies to no avail. Oddly enough my old Radeon 4850 hasn't shown any problems like this.
I am starting to suspect that there is something wrong with my environment that is causing these cards to fail. Is it possible that my PC isn't drawing enough power from the outlet? Or could there be something in my case that is frying the video RAM?