When this happened I was using the Via SP13000 motherboard. I left the car at a shop for an entire week and when they went to start it up, it was dead.
It would survive 3-4 days at the most. Now I'm using the Jetway 1.5GHz board, but have yet to test this and I'm not interested in doing so either :). I have left it over the weekend and it's fine. I don't know what causes this, but it does happen. I'm using the M2-ATX as well. I know it's not supposed to, but it does.