I have used my microwave "flawlessly" for 3 years. In the middle of use one night, the breaker tripped. I reset the breaker several times, kept tripping. Once the breaker is reset, microwave appears to be fine(light and clock come back on). Then when you press start... it trips again. Could this be the microwave or the breaker ?

Have you tried plugging the microwave into another outlet, or plugging another appliance into the microwaves outlet? Might narrow down where the problem is...

A microwave pulls 10-15 amps. It needs to be on a circuit by itself. I have the same problem with mine if I don't turn off everything on the circuit, even a big light fixture. You could also have a weak breaker that needs replacing.