Having had two PSU's fail the 'short' is obviously in the motherboard. I would be inclined to dump the motherboard and fit another one since PSU's can be more expensive than some MB's. You say that the MB is two years old, it would be fair to say that the MB is much older than that !
You may find that today's cheaper MB's such "Rock", manufactured by ASUS could have a higher specification than the one you have fitted and , this could be cheaper than another PSU.
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