A. It can only slow down
To me it sounds like a system error, or drivers error or some sort of spyware/maleware that already caused damage to system files.
Ways to check it out is:
first way is trying to do System restore after you boot into safe mode, if that fails:
boot into safe mode, reinstall video cards drivers and stuff like that.
Then, scan for spyware and maleware.
If all fails I suggest to re-install the O.S.
If that doesn't work then you should start thinking about hardware damage or maybe not sufficient PSU... could be all sort of things though