Wally, please run this program to see if the system is fully stable:
linx:
XtremeSystems
it is one of, if not, the heaviest stress test program out there, and if the test fails after a bit, then the system is unstable. Prime95 and SuperPi do not come close to stress testing compared to this program. It was how I was able to tell if my 4ghz OC on my X6 1050T was working or not.
Also, do you ever get any BSODS? Another thing to try is
Memtest and give it a run.
If you update your video drivers and that does not work, then might I suggest doing what frankbooth suggested and testing each card individually, and if neither of them work, then last thing may be the powersupply. What PSU are you using? Also, is your bios fully updated? are chipset drivers and alike fully updated as well?
Edit: One last thing to give a shot, and it is a tad bit extreme, is to re-install windows or revert back to a time where you did not have issues. When you uninstall drivers, do you run driversweeper in safe mode? Cause conflictions with old drivers and new drivers do happen.