Small graphical artifacts appear only in certain games

I got a second-hand PS4 Pro in near-mint condition. I took it apart, cleaned it (not too much buildup), and repasted it for good measure. Some games run perfectly smoothly (Spiderman, Asscreed Odyssey for example, slightly older ones I guess), but it was when I booted up Ghost of Tsushima that I noticed these black flickers across my screen, not horribly game-breaking, just a visual bug really, and not all the time, but it still concerned me. Recently downloaded Kena, and it started showing the same (on two different screens too). Is the GPU on its way out?