PC 'softlocks'? randomly

I don't know what to call it, but it doesn't 'turn off'. It goes black for a second and then the monitor says it's not getting a signal anymore, the fans and lights inside it are still going, the PC is still by all rights 'on'. Just unresponsive. It could be in 15 minutes or 6 hours. I'm not 100% sure if what I'm doing on it at the time matters.

Things I know/have done:

• I know for a fact it's NOT a heating issue.
• I know for a fact it's NOT a monitor issue.
• I know for a fact it's NOT a HDMI/wire issue.
• I updated my graphics drivers because why not.
• It still turns off in Safe Mode.
• I swapped out some of the wires connecting the PSU to the graphics card and motherboard to see if they were the issue. They weren't.
• I checked if it was just the monitor being unresponsive by having a notepad selected and waited for it to 'turn off'. When it did I started typing random things on the keyboard and CTRL-S to save the file. When I restarted it, it didn't save. So other things are becoming unresponsive as well.

I'm starting to think it's either the PSU starting to fail or the Motherboard. However I don't know how to check the PSU since I don't just have another one around to test it.

Not sure if there's anything else I can troubleshoot besides 'buy new parts and hope for the best'.