Why does my PC have fluctuating FPS drops on Fortnite?

My GPU: AMD R9 200

CPU: AMD phenom ii x4 965

Windows 10 64bit

I mostly play Fortnite and have about 140 fps on lowest settings but sometimes it drops down to 20.30 fps. So relatively unstable, so the question which component should I renew?

CPU, then broke synonymous motherboard and Ram new

Probably the CPU gets too hot. What do the temperatures say?

I would be the Cpu beginning, since this is already quite old. But I would recommend you to wait for the 3rd generation of Ryzen processors that are coming now.

And why a new motherboard?

Because a new or newer CPU has a different design or a different socket. If you want to become an AMD you need a motherboard with AM4 socket nowadays