I have had this EXACT same problem. Comes and goes. Since I've moved, it hasn't been an issue. I think it may have been dragged down by the house wiring, even though wall voltage tested okay. I took it to an all day fund raiser event and had zero problems as well.
The first time I experienced the problem, I cranked on the +5 pot.
The problem eventually came back, but unfortunately the pot was maxed out.
I have put my DMM on the test points and observed the voltage at the cpu when it was at the worst. The voltage would stay around 4.9, then start to drop to about 4.8 where it would begin to reset. Some days it would behave perfectly fine. The whole thing was just so...random.