I've had this problem before, and it was none of the things that were said above.
Back when I was running a PhenomII 965 with my 6950, I started to have this problem. I have no idea when it started. I had tried everything short of re-installing Windows, well I thought I did.
What did the trick? I tested my RAM and found a bad stick.
Moral of the story, it can be the things you do not expect it to be.
Ram was in the wrong slots.
+ I upgraded to the 6850. Thank you for your suggestion.