Built my first PC. I need help :(

I recently built a mostly new $400 budget PC with the help of a guide from OzTalksHW on YouTube & it came out really good, or so I thought…

I’m having problems at the moment, I’m getting significant frame drops in pixel games like stardew valley & fields of mistria they’re very playable, but every once in a while I get some frame drops, enough to where I notice which shouldn’t happen? but I’m no expert. Anyone out there know what the culprit may be? Could it be the CPU?

The CPU & GPU were second hand.

Here are my specs:

Ryzen 5 3600 @ 3.6ghz

Gtx 1070 8gb

16gb DDR4 3200MHz