After having issues with (and having to RMA) a GTX 670/Extreme4 combo I did some research into that particular combo and I think there are many users with instability issues stemming from the two components. Earlier suggestions have been good, but the one I'll throw out is to update the mobo bios. Even new Extreme4's come with bios v 2.00 which doesn't support gen3 pcie bus speed. So first I would go into bios and go to advanced tab -> north bridge config which should tell you your current pcie bus speed (gen1, 2, 3 or auto). If it is running at gen1, that could be your problem. Cheers