Why are there squares on my screen?

Since a few weeks my MacBook has been running slower and crashing more than usual. I couldn’t play games anymore for example because after a few second the screen would freeze completely and I would have to restart my Mac. Along with that, sometimes squares have been appearing on the screen. During one of these episodes I can’t click anything, it usually lasts from a few seconds to a few minutes where is have to restart. Sometimes my Mac crashes completely because of this. I ran a diagnostic on my Mac a few times but these came out normal. Today I also ran a virus scanner which didn’t point out anything abnormal. It’s only getting worse and worse, what could be the cause of this?