When in doubt it is usually the power supply that is the culprit. Unstable, able to overclock parts on a different power supply, and a completely different board with the same power supply didn't solve the problem. If your CPU and GPU aren't receiving stable power they may be self throttling, or having lots of errors. (You can check Windows logs for evidence of errors)
If you friend is willing, give his power supply a try on your equipment and see if the problem goes away.
If you have a voltmeter handy, see what the 12V looks like when the computer is under load (jam the leads into any spare molex connector from black to yellow). 11.4 - 12.6 is considered in spec, but anything under 12 is not that good for a decent power supply such as this one. If it is still in warranty, RMA it if it doesn't look right.