Why are my Fps in Fortnite partially lured to 30?

I play in my free time, as far as time is fortnite.

During the game, I notice that my fps sometimes stay longer at 30Fps and then go back to continuous 60fps. My specs

GTX 980ti

I7-6700k

32gb ddr4 memory

Well I know that I should not have any fps problems with my PC, but I can't really explain why or how the Fps in Fortnite on 30 are lured and that only for a short time and then again not?

I assume that there are background tasks running on your PC slowing down your game, etc.

Check out what you have in the background while you play.

I'm pleased about the "most helpful answer" or a "thank you".