Have you tried underclocking your card to the stock 680 specs of - Base Clock: 1006MHz and Boost Clock: 1058MHz? You can use MSI Afterburner or EVGA Precision X to do it with. After that, try running Valley and BF3 again. There's been 3 revisions to the graphics card. If this test works or doesn't, you'll need to follow the steps here to get a replacement.