First off this could be a nVidia driver problem. Secondly in nVidia control panel there should be an option to OC, I don't have a nVidia card so I don't know what its called but like what you said, I cant OC in Msi Afterburner without ATi Overdrive turned ON. So look for that option. If you do manage to get back to normal clocks, its probably not the best idea to OC a mobile graphics chip. Another thing to try is to roll back drivers, use older ones.