It's probably too late, but you should have a real good look at the motherboards capacitors. They have been known to fail and are especially susceptible when subject to excessive heat for extended periods (as in small cases with minimal ventilation). If you notice any bulging and or what looks like tiny specs of rust on their tops this would be an indication of failure.

The reason I mention this is that I have experienced similar issues in the past with a motherboard that had a couple bad capacitors.