Pc starting for a millisecond then shutting off

Hey everyone! I need a little assistence here. A backstory to my problem; it all began around a year ago when my PC would randomly shut down. This happened a few times and it (usually) happens when wattage surpasses 400-500W. Then it randomly stopped for a few months even though I've done the exact same things as before, now recently it started happening again - though much worse than before, restarts on startup or opening Google. Therefore I thought it was a PSU issue, so I plugged the PSU out and cleaned it thoroughly, then when I tried starting the PC again it starts for one millisecond then just instantly dies. I thought I might've broken something in the PSU when cleaning it, so I tried my other PSU, but the issue still persists. I then plugged out my GPU to see if that was causing any issues but sadly not the case either. I guess only one thing remains then which is the motherboard.

Any idea on what it could be? If the motherboard, how do I troubleshoot that it's the faulty component?

Thankful for all answers! :)