I'm no specialist in the new ATX pinout but you mentionned the PSU turns off after a few seconds. Have you tried just connecting pin 16 (PS_ON) to ground to turn the PSU "ON" then disconnecting it so it should stay powered.
AFAIK you just need to have the power on pin shorted to ground for a millisecond to turn on an ATX power supply?
Tought it might work.....?
