HP COMPAQ DC7900 - Won't POST, good PSU, board has power and green light, no beeps

Looking at a pc for someone, it's an ultra small case with proprietary psu. System will start to boot when plugged in, psu fan and case fan receive enough power to start spinning but stop within 1 second. System board has green light, no blink. Case switch seems to be functioning as well.

Checked every connection, cap, unplugged everything, popped cmos battery. It did happen to start randomly while plugged in for 20 seconds after sat aside for a few hours. Just long enough to hear it turn around and see it cut off again. And then it went back to no post.