turn off and back on

I'm trying to figure out if this is a logic board or PSU problem. I have an AOC C27G1 that recently started turning itself off (even without connection), at first it was about a week between the shutoffs but got gradually worse until you could barely see the AOC logo when starting it up before...

My PC was fine but I bought a new case fan which I connected to my motherboard. While I was playing a game, my pc lost power unexpectedly and started to turn on and off repeatedly. I switched it off, unplugged my power cable, disconnected the fan and drained the power by holding the power button...

The screen will randomly go black for literally a couple seconds and then come back as if nothing happened and then won't do it again. Happens sometimes after I close a game too. I'll exit a game and go to the desktop and then after I see the desktop the screen goes black for a moment and then...