Have not reproduced it via various load tests and Dirt 3 gaming. But both times it seemed to happen when computer was sleeping and then woken up. I did test setting sleep to 1 minute to force sleep, and did not reproduce it that way either.
I also had to reduce what I thought was a previously stable OC to prevent issues, but I am not sure if that's a driver difference, or a card issue...trying to reduce variables, by first getting back to a stable overclock (so far, 860/1350 on the 6950s seems stable with max voltage allowed by Trixx.)
So nothing definitive to report.
On the bright side, Dirt 3 finally runs with decent frame rates ( > 100) except in the rain Rain definitely seems to bring fps down by a lot.
Ok, no progress other than to suggest it's not a card problem, it's a driver issue.
I ran 3D mark extreme looped for over 30 minutes, and it ran fine. Played Diablo 3, seemed fine.
But when I left PC for long enough, video for games simply does not work. I have to disable crossfire. And I can't always enable, because second card won't show up in Catalyst I think. A reboot is then needed.
12.4 solves. It also gives expected fps in Dirt 3, unlike some of the later versions, which was something 12.8 solved ( been wracking my brain trying to understand how dual 6950s only did 80 fps on the in game bench.)