Let me repeat advice I've given elsewhere: download the free trial of Passmark's BurnInTest software and run it for a while. It doesn't just identify overheating issues (which it did for me), it also tests various subsystems of your hardware and the OS.

It's free, doesn't take a lot of effort to use, and will at least give you some confidence that the problem isn't, for example, DRAM that's gone bad.

Let me add one more option, but this one isn't free:
I found that putting my PCs on a sine-wave Uninterruptable Power Supply (UPS) made them a lot more stable.
A good UPS will block spikes from, for example, an AC unit switching on, which spikes might get through your computer's PSU and cause a soft error.
I'm using CyberPower 1325VA units on all the systems in the house (four of them) and have no regrets.

(I use smaller units on my router and cable modem, and have therefor been able to keep playing FFXIV even during short black-outs.