my money is on your shorting out somewhere as said "repairman" stated.
and what he means is that some where along the line a "hot" lead (the path/wire that carries the 12v-5v input current) is touching metal in your case, why is this bad?
Because unlike a house 12v DC relies on a single path.
Ac input-> power supply-> 12v or 5v current -> (place device here) -> ground loop -> power supply
when the hot lead is touching the metal is basically the same thing as touching 2 120v AC "hot" leads together ( better known as arcing a circuit) and this can fry anything that is now touching metal, now you may not feel this when you touch you case as the PSU is active because it takes some power at 12v DC to be felt by a human.
when he said "shorting the motherboard" you probably didn't install some/all or the risers. if so you definitely need to.
Now take everything I say with a pinch of salt, I am by no
means an electrician but I've dealed with enough of the simple wiring of life (both AC and DC) that I have a faint basis in electron theory.
You can troubleshoot this with a multimeter.
this is a multimeter if you didn't know.
like I said I'm no pro but it's just my $.02