GE Spacemaker over-the-range Microwave repair?

I have a GE Spacemaker over-the-range microwave. Lately it has started giving problems. Sometimes it works and sometimes it doesn't. When it works it does heat up the food. When it doesn't work, the light inside the microwave comes on for a second and then it just makes a humming sound. Sometimes it starts as normal but after 2-3 seconds the light turns off and it again just makes a humming sound. Can anyone give me a clue as to what might be wrong with this microwave?