I have a customer with a GE Microwave. I went and looked at it, and it worked fine. Boiled water and popped popcorn with no issue. I looked inside, checked connections and said I couldn't find anything wrong with it. They use it later that night, and say it shut off. I come back next day and it works fine. I check everything I can think of. Could it be a thermal cut-off or the control board?
I don't want to just start changing parts, but it's hard to diagnose something when it works fine when you look at it.