So basically on mine (same motherboard) the switch circuit is trashed. If you try to switch it off with the stock power supply or a new one it just resets over and over very fast making a bad clicking noise. Even with a new ATX power supply wired (just tried it ) same thing .. basically verifying the problem is on the motherboard and not the supply.
This could be bypass caps (at some point I'll get around to recapping) but I actually doubt it because all the caps that I've tested in this board have normal esr and capacitance readings. This is actually a problem because if you apply power in an "off" state the system firmware for some reason thinks you have a laptop of some sort ( oddly ) reveling a bunch of options inside the configuration utilities on page 3 and page one that look like corruption. If you place a jumper between these pins and bypass the switch (either by using a mechanical switch or my soft power switcher project https://github.com/lindoran/ps1pwfake) the computer boots up realizes the nv ram corruption and everything just works. I did go In and turn off apm completely on page three as well :
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It does appear that the nvram stays correct even after you remove the jumper if your not comfortable leaving it in there forever like I did.
Wish we had schematics for the switch circuit and the apm stuff of the main board but heh - ibm