I'm one of those people who will go down the rabbit hole and not come back until I've figured it out, else I would never trust a flaky system ever again. This happened with one system where memory was failing, but it was in a very specific region which would coincidentally be where the OS stored filesystem structures, so the filesystem would become corrupt after a certain amount of use. Since it didn't feel or seem like anything memory related, it took weeks of trying different things until I decided to see about the memory.
On most computers, holding the power button for five seconds turns off the power, whereas a hard reboot might be similar to someone pressing the reset button. If it were me, I'd try, in order, checking that you have the correct version of your BIOS (I accidentally loaded the latest BIOS on a system with a Ryzen 2600, and it made memory transfers unhappy), using another power supply for a while, running another OS, like NetBSD, on an external drive and doing lots of test compiles, clocking the RAM one or two steps slower, then removing all but one DIMM at a time and testing with just one DIMM.
On most computers, holding the power button for five seconds turns off the power, whereas a hard reboot might be similar to someone pressing the reset button. If it were me, I'd try, in order, checking that you have the correct version of your BIOS (I accidentally loaded the latest BIOS on a system with a Ryzen 2600, and it made memory transfers unhappy), using another power supply for a while, running another OS, like NetBSD, on an external drive and doing lots of test compiles, clocking the RAM one or two steps slower, then removing all but one DIMM at a time and testing with just one DIMM.