both the downclocking and crashing in physX testing sounds like a driver issue.
make sure you have the latest drivers from nvidia
uninstall the driver(s)/software in Control Panel\All Control Panel Items\Programs and Features
then for added benefit reboot into safe mode and run driver fusion
then reboot to install the latest drivers.
also temporarily turn off windows update. it will download and install it's nvidia driver after you uninstall what you have now and before you install the latest. turn it back on after you're done.
now this just maybe a start or it can be a final solution - but its best to make sure.