Asus Crosshair dead.

My wife recently purchased a wonderful I pod touch. As I am currently writing from it, I have a big problem. So I plug the iPod in one evening to charge it, and my machine comes to a halt. It will turn on for 5 secs and shut off. If I unplug the 12v motherboard plug it doesnt shut off. so after all the testing I determine that my motherboard has failed. So I purchase another cheaper ASus board put in a new psu. Then 4 week later I plug in the iPod as I usually do to sync it. My machine now does the same thing. Any recommendations? its obviously some sort of USB problem. Something has shorted and won't let it boot. But in both cases disconnecting the 12v psu connector from the Mobo will let the machine stay on. I'm desperate for an idea.