Lmah :
Is your GPU getting hot? If it is it could be down clocking. I would check your temperatures and GPU's speed to see if it is being down clocked. In CS:GO you should probably just cap your framerate at 140 anyways. There's literally no benefit having it higher.
I have a R9 280x toxic and I am only having this problem with CS, so I think I can rule out overheating, I will lock my FPS though
😀.
Ill give my temps a check just to be sure too.