I've had some stability problems with a new setup of mine. It has to do with me just starting some sorta video, and then the computer decides to move in slowmotion until I reboot (don't happen very often, though). I thought these tests might give me some answers, maybe not?
While worthless for benchmarking, I find the 3Dmarks (06 and Vantage) good stability test programs. I think though to really max out the card furmark is the normal choice.