BubbleMuscles

Firstly, check if something is hogging the CPU. Maybe you have something, which takes up most of the processing power of the computer running in the background. Lastly, try cleaning the cooling system of your computer. When CPU is very hot, your computer may slow it down in order to prevent possible permanent damage to this component at the cost of the performance. When using laptop, make sure that you don't block ventilation holes on the device.

@[LKW Tr.] Alis* before that happen i had 60 fps, but when it started i had about 15-20 fps 
@Titanic4ok i will check this later. but while i play games , i dont have any other apps/games open

I had exactly same problem in almost every game... Everything was good and than from sudden GPU usage dropped to something like 5% for like 30 seconds, during that time games were unplayable. Reinstallation of drivers solved it for me.