If the receptacle works, then check the plug from the microwave. Sounds like you could have a short in the wiring, check the connection of the plug in the back of the microwave. Make sure no wires are bare, check all wire nuts, make sure they are on tight and both wires inside each wire nut is in contact, best way is to just redo the wire nuts. If this doesn't solve it you could have a short inside the unit itself.