Acting very erratic had to reinstall OS still problematic

Acting very erratic had to reinstall OS still problematic. I’ve been running blender on this computer a lot and it almost seems like it’s better with a fan running behind it after reinstall. But it is acting very bad. Sometimes it will say missing operating system. I got it from university repair store. They took out the DVD. And put a SDD and a hdd terabyte. It’s been working great until lately. I don’t know if this is logic or or what. I don’t think this has fans? Anybody have any ideas of what I could tackle to fix this …..when I run diagnostics it said the hard drives are fine. Everything is fine. Not. Lol

wallly