Un-install the drivers, then boot into safe mode and run Driver Sweeper to make sure all the Nvidia Driver files have been removed from the system.
Reboot into Windows and re-install the "Newest" Drivers and see if your problem still exists. If you still have the problem you should remove one of your cards and test it all by itself. You should also try the "other" card all by itself. If the problem still exists with both cards running by themselves then the problem is probably with the Nvidia Driver or a conflict with another system driver. If the problem goes away then it may be a hardware issue.