Asus motherboard intermittent power on problem

I have an Asus A8N-SLI Premium and Antec Smartpower 500 which have been working reliably since February. Since upgrading the case I have an intermittent power on problem. I disconnected everything but the processor, fan, ram and video. It's out of the case resting on books. The green LED on the motherboard stays lit consistently with psu plugged in and switched on. I've tried clearing rtc, reseating connectors etc. I've made it start intermittently with the video card and without. It appears to be totally intermittent - mostly it doesn't work. It seems more likely to start if the psu is switched off and disconnected from ac for an hour or so. I've tested the psu with a P3 motherboard (not dual 12v) and it works consistently. I feel the motherboard is at fault. I've emailed ASUS and Antec. What do you guys think? Thanks.

 

Oh yeah. Upon shorting pwr pins, the symptom is green led stays lit, and nothing else happens. No CPU fan movement, no dimming, clicking, video lines, nothing. Unless it decides to work and everything spins up, turns on etc. Thanks again.

 

Another observation

When it decides to work, it will work repeatedly, only if the power supply remains plugged in and on. If the power supply is disconnected or off, intermittent trouble returns. The CMOS battery was tested good.

 

Unfortunately, I don't have another dual 12v psu. I have the motherboard running right now. I've been shutting down and restarting periodically to prove that the problem occurs only if the psu gets turned off. It feels like a CMOS issue. I'm just not sure without an extra psu and mb. Thanks.