I've had two cases for this... the first was actually a USB plug on the motherboard. Something was touching that shorted it out (didn't care to mess with it as I had five extra, hah, so ). Then there was a friend who put his motherboard on with no buffer from the case. It worked amazingly enough, but the slightest
movement caused a reset (I wonder why
). So keep an eye out on the various metallic components, as those could be the cause (they get mad easily, when touched improperly).