Question Computer performance keeps fluctuating and I don't know why

Apr 4, 2019
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For example, I was able to run all settings on high with no stutters at 80 fps. Then the next day Im running medium at 60fps with microstutters. I knoe my CPU is bottlenecking because it's not able to keep up with my GPU, but I don't think that explains it.
Specs:
CPU: AMD fx-6300 (no oc and stock cooling)
GPU: Nvidia GTX 1660TI