I had the same problem on my computer.
The only way i got sli working was to add my old 9800GT as dedicated physx card.
It's strange, as everything works fine by themselves but when i enable sli without the 9800gt the driver stops working.
I have tried every old and new driver, reinstall and other setting in my bios. Have asked Gainward for support(all my cards are Gainward) but they haven't reply. Looks like i have a ghost in my computer
Think i have to get a lighter physxcard if i hvae to continue this, becouse my PSU is only rated at 750W.