It may be an issue with cards from ASUS or MSI as I have had factory OC'd cards from both BFG and EVGA and did not experience any issues with either. Actually, I was able to further OC each of the cards. I currently have an EVGA 7900GT CO Superclocked (OC'd at factory to 550/1580 and then to my OC of 575/1600) for about two years now with no problems at all.
I highly recommend EVGA products and will probably continue to buy them exclusively unless HIS comes out with something really tasty from ATI/AMD.
Are you sure it's not a Vista? Or driver issue?