Big FPS Drops

Since a few days i have huge FPS drops when playing, they appear to be random, not only when much is happening at the time.

I never had problems with that, normally i have constant 140 fps, now they crash to 40 sometimes.

I already set Priority to high in my Task Manager for LoL, changed my PCs Energysettings to high and activated the gaming mode.

Do you guys have an tips? Thank you