Stan_Lite, with your GTX670s what NVidia driver versions are you using? I'm still on 320.49 because of the problems later versions caused with Folding@Home but the newer ones include CUDA6 which is supposed to help with GPUGRID. Any views?
Currently running 335.23. Seems to be fine, although I've only had it back on a few days so it's hard to tell. I shied away from newer drivers after several fiascos with F@H and new drivers but, so far, I haven't had any issues with GPUGrid.
Updated the drivers in my main rig (2 X GTX690) at about the same time and it seems to be fine too. It seems as if BOINC is more resilient to driver updates than F@H.