I dont know if its a problem isolated to the geforce 7950gx2 but i had the same issue and it was due to my card not liking 4gb of ram. I took 1 out and tried and it worked flawlessly. You might want to give that a try and see if it helps before troubleshooting forever
The issue only came about when trying to switch to multi-gpu from multi-display. it worked fine on multi-display.
EDIT:
Damn nice specs BTW