Could be something getting heated up like Ross said, shutting down, & then reseting itself after it cooled off.

I used to see that happen a lot to electronic equipment at my former employer's (Boeing) factory. We'd call an electrician in to fix it & when he got there, it was working fine. He would want to know why in the heck were we calling in bogus trouble calls & bothering him. It was very frustrating, it could take WEEKS to diagnose & fix the problem.
Regards
Bill R-4581