PC randomly black screens, USB dies, fans keep spinning

My ~9-year-old PC has started randomly hard-freezing:

hard-freezing:

- Screen goes black / no signal

• Mouse turns off
• Caps Lock stops responding
• Fans keep spinning
• Sometimes it also boots with no display

The only way to recover is a hard reset / full power cycle, like, hold power, switch off/unplug, then power it back on. After that it usually boots and works normally again for hours or even a day before it happens again. I already did a completely clean Windows 11 reinstall and the issue still happens.

Specs: ASUS PRIME B450M-A Ryzen 5 2600 16 GB Corsair Vengeance LPX DDR4-3000 (2x8 GB) ASUS Turbo RTX 2060 6 GB Corsair VS650 PSU WD Blue 500 GB M.2 SSD WD Blue 2 TB HDD

I took it to a repair shop and they ran software/stress tests but found nothing obvious. I’ve asked them exactly what tests they ran and for how long. What would you test first for an intermittent issue like this?

My main suspects are PSU, RAM, motherboard, or GPU. Would you run MemTest86 overnight, test one RAM stick at a time, or start by swapping the PSU with a known-good one?