just test everything.. process of elimination. that's how you fix a computer
-reinstall windows. that eliminates any software issues/virus etc. the usb boot is a great idea because it also eliminates your hard drive from the list of things it could be.
-test each stick of ram individually
try those first since they are free
after that you have psu, mobo, gpu, cpu.
gpu unlikely the problem but its easy to test using onboard gfx or another card, cpu very unlikely its usually overheating to worry about here or builder error.
psu has a decent chance of being it, if you can try it with another psu. and the mobo which is last resort to me because its a pain to rebuild your system.. but maybe its configured wrong. or since its a pretty old pc you can get one dirt cheap
If I had to guess, your processor is overheating as those quad cores run hot and you probly didnt change thermal paste in 10 years, but that is literally a guess.