It's the ATI driver, 12.8 is broken, 12.7 was broken.
12.6 & 12.3 are the last versions that worked correctly. Too many bugs in newer one to be worth using.
There is always the possibility you might have slightly screwed the card by unlocking it.
("slightly" as it can be undone & shouldn't cause any physical damage)
Pretty much a lottery doing so, and the odds of having a 6950 with the same number of fully functioning stream processors as a 6970 is extremely low - there's a reason the chip ended up on the card it's on, and given that there's not been a shortage I can't see ATI/AMD re-purposing fully functional highend chips onto a lower end card.
But at the same time you may have scored lucky. None the less scrap the new drivers, they're shite. Stick with stable ones.