System Will Not Start

I have MSI K8N Neo Platinum motherboard that I installed as a replacement board about a month ago in an existing system. For several weeks this board worked as expected, but beginning two weeks ago I began encountering several strange problems.

First, two weeks ago I turned the computer on after about a week of inactivity. It powered up but did not post and had no video output. In response I pressed the power button and shut down the system hoping it would restart, which it did, but only for about one second before powering down. After reading several posts here I checked that the stand-offs were positioned properly and for shorts, but found none. I performed the normal card/drive/memory removal trouble-shooting and eventually found that if I removed power to the graphics card, the computer would start and stay running (with the expected error tone). After letting it run like this for about 10 seconds, I restarted again and the computer booted normally.

The next week went by without problems. I used the computer every day that week. Then last week I didn't use it for five straight days. I turned it on yesterday and it did the same thing as before, a power up but no post and no video. And so I attempted to restart again, but this time it didn't respond at all and the previous fix did not work.

I thought it might be the power supply, so I swapped it out with a spare one I have with no change. Next I checked the voltages on +5VSB (5.14V) and PS_ON# (also 5.14V). And so I attempted to hot wire the PSU by connecting PS_ON# to COM. The PSU powered up as expected starting the fans and drives (but not the main board because it obviously was not connected). I checked PWR_OK which also registered 5.14V. So I concluded that my PSU was working.

When I plugged in the MB again the system flashed on for an instant and then immediately shut down.

I next moved my attention to the POWER SW pins. They registered a voltage as 1.24V across the pins. I thought that my power switch may have been bad so I attempted to jumper the pins without any result.

I then thought that since this only happened after long periods of inactivity, maybe the battery was being drained while the system was off, but it was reading 2.84V, which seemed reasonable to me.

And this is where I'm at. I'm not sure where to go from here.

Does anyone know what the next step in the start up is after the POWER SW pins?

Since this only occurred after long periods of inactivity, could that have something to do with the start up? Could it be an empty capacitor or something? And if so, how would I find it?

Any other ideas?