Cpu usage drops causing fps drops

Jun 6, 2018
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So my cpu usage is around 70% in a normal game on normal graphics on games like Fortnite or Overwatch.
But then it drops to 40% when i'm having a FPS drop.
I already bought a cooler, helped a bit.
I have good FPS for about 20-25 seconds, FPS drops stay for about 20 seconds and then it goes back to normal FPS

Specs: PC- Acer Aspire ES1-521
CPU- AMD A6-6310 APU ( 4 core's )
GPU- Radeon R4
RAM- 8Gb