Okay, got a Gigabyte P55-UD3R motherboard which I've been using with a 450W Corsair power supply for the past few months. It was stable, but I was uneasy feeding my Radeon 4870 with one PCIe power connector and one 2IDE-PCIe converter, so I ordered an Antec 700W power supply in a package deal with 2 more 4870s. I finally got them yesterday and last night I swapped power supplies.

When I tried to turn it on, I noticed that my CPU fan would take one partial turn and then stop. No problem, it's connected to the CPU_FAN connection on the motherboard, and I figure that it only runs when the temp is up. About 15 seconds after it stops, it starts back up again briefly, then the whole system shuts down. I recheck all my connections, try a few more times, but it always behaves this way.

Unsure what I've done wrong but without the time to keep dicking around, I swap back in the Corsair PSU. I turn it on, same problem. I unplug everything else and try it, still no go. I take the motherboard completely out of the case to head off any shorting problems, try again, same exact behavior. Right now, I've switched the power supply back to the new one since I'm pretty sure the problem is not a defective power supply, but I can't figure out what exactly it is. I have both the 8-pin and the very long power connector plugged into the motherboard firmly, so it's not something that simple. It strikes me as a blown motherboard, but I'm not sure how I blew it just switching power supplies. Anyone have any clues what causes this startup behavior?