any time i get strange shut downs or sporadic power issues i swap the PSU, remove all items from the case and boot the PC on an anti static pad to eliminate the possibility of shorts to the case. aside from that it could be an issue on the PCB itself. another issue i have had is a bad ohm level on the power button, ive seen this cause such problems. do not power on the computer via the front panel connectors but rather unplug the connectors and use a key or screw driver to close the 2 pins for the power button on the MB.