It could be either the PSU or mobo. I'd take the PSU to any local computer store and have them check if it's faulty (since it's so simple some do it for free). If that is the case, obviously replace the PSU. Otherwise, I'm leaning towards a faulty motherboard. With computer rebooting in loops it has to be the PSU or mobo. It could possibly be faulty RAM, but I think that would be the 3rd option after the PSU and mobo.
Since it's late (I'm assuming you'll check this soon) you could run Memtest86
for your RAM. If it's faulty it will save you a trip to the local computer store tomorrow and/or replacing the mobo.