FPS lag after replacing harddrive

Hey guys, a few months back my GMod used to run perfectly, 60+ FPS.
Then harddrive died and I had to replace it and since then GMod has been running awful, an average of 22 FPS at best.

A strange thing is that if I minimize it and then go back ingame FPS goes to 60, but then gradually goes back down again to 22.
I’ve tried upgrading/downgrading video drivers but that only affects the first startup, after closing and reopening the FPS is back at 22 again.

And help for this?

Specs:

Windows 8 64-Bit
Intel Core i7-4700MQ @ 2.40GHz
8GB RAM
GeForce GT 750M

Hm, a day later and no response… no-one can help with this?