I'd get the "dead" psu's out and test them somewhere else first, just to make sure the PSU's are actually dead. If they aren't, then you probably have a duff connection or bad track/joint on your motherboard, and the process of changing PSU is temporarily fixing it. Like the previous poster said - check all your connections.
Its also possible that the motherboard is'nt giving a good "Power Good" signal back to the PSU. Do the LED's on the front of the box flick on momentarilly when you first connect power/switch on with a "dead" PSU?