Need help with PC! (Computer experts pref please)

Hello!

So lately my pc keeps getting black screen (same as putting pc to sleep or locking it) after a while, the period always varies, there's no specific time it will happen, it can go from literally a minute from startup to 4 hours which I believe is the longest it has gone without getting it. I assume this doesn't happen because of overheating since as I said it sometimes happen after 1 minute from startup so perhaps a PSU issue? I've already cleaned my entire pc from dust using an electric plower and installed a new OS version and have the latest gpu drivers installed, also my temperatures are normal. https://i.imgur.com/7l5StNv.png

Any idea what could be causing this? Thanks in advance!

What brand/model power supply do you have? Also dang that's quite a dated PC.
Also I recommend running a MemTest https://www.memtest86.com/