My desktop with ASUS P5L-MX motherboard does not immediately want to boot up whenever it is completely shut down. When the switch is pressed there are no visible lights, fan motion or anything. It happens every time. Then if I re-seat the power cable, and sometimes remove USB devices, then try again it suddenly powers up again. Doing a reboot without completely shutting down is no problem at all. Is this the power supply or motherboard? How do I narrow it down?