Hmm, what you describing is interesting, try to update DirectX drivers, they sometimes go craz, also you have some kind of GPU computation program installed - Boinc, Bitcoin miner, or something else (could even be some virus which is doing gpu computations for something), it would explain why you have high fps after driver update (because the update will kill all programs using it), and its back with the restart. So If you are not aware of something, run antivirus scan. Possibly you can use eset sysinspector for quick overview what is running in your system (if you use it, slide risk level to 6, and manually look on what is left in the list), you can even use it for comparing of what is running before and after driver update (just save the log before update and run again after update and use compare).