If you've ruled out viruses, and hardware problems, It might be worth running the System file checker, which will at least check that the windows OS files are intact, and not compromised.
To do this, I think you need a CMD window with administrator permission, and then run SFC /SCANNOW. It takes a wee while to go through and check all the system, but I think it might be worth trying.
https://support.microsoft.com/en-us/windows/using-system-file-checker-in-windows-10-365e0031-36b1-6031-f804-8fd86e0ef4caThen maybe after that, it might be worth running another virusscan.
TO give you another option, I sometimes use trendmicro's housecall if I'm fixing machines with no AV.
https://www.trendmicro.com/en_gb/forHome/products/housecall.htmlIt might also be worth looking to see if it is a rootkit messing with your system, and I found this one that malwarebytes does.
https://www.malwarebytes.com/antirootkit/I hope some of these tools might help you find the issue.
Also the other previous advice you've had about checking the PSU, Memory, Keyboard, different OS etc, is all solid advice. Problems like these are tricky to pin down, and it can take a lot of time to trace it. Just follow through things logically though, and you should find the root of the problem.
One more check I will add in, if your machine is a few years old, don't forget to check the BIOS battery. Its an often overlooked thing which if it is low, might be low enough to cause niggles, but not enough to loose the BIOS time or trigger battery warnings. So it's worth checking it is good with a multimeter. Just remember to save your bios settings first, as you may have to redo them.
Good luck in finding the fault.