Only thing I can come up with is......
Are you using separate power cables from the PSU to the GPU, some cables can be daisy chained but this is not a good idea for power hungry cards like 6950x. You have to make sure it's getting all the power it requires from the PSU.
While in your game, bring up the performance overlay (Ctrl+Shift+O) and see what kind of wattage the card is using and how much it fluctuates, it will fluctuate as the game scenes get more or less demanding as you play.
Good Luck, I hope you find your problem.
Regards: Jack