I built this comp a little over a year ago, and have been using Linux from about 2012. This has happened with Mint, Fedora & OpenSUSE. My computer freezes (can’t even SSH in) randomly; it can be in gparted. In a game, or watching YouTube in Firefox. Ran 1 pass of memtest86+, no problems. When it freezes, I can usually move the mouse & sound often keeps playing for a minute.
This sounds like a hardware failure. Power supply testers can be had for under $20 USD, a logical place to start looking. Did you look for reviews for the power supply you used? One pass of memtest often isn’t enough. It should be run overnight, at least 4 hours if not many more.
I bought a Gigabyte motherboard that uses DDR4 last December. After 5 months of occasional use two RAM slots went bad, no POST if RAM was installed in either one, so it had to be replaced via RMA. Check to see if your motherboard model has a failure history.