When I have used my microwave for a prolonged period of time (defrosting then cooking a large amount of ground hamburger, etc) it changes from a normal working sound to a lower pitch hum and stops cooking altogether. If I let it 'rest' for a couple of hours it starts functioning normally again. If I don't let it 'rest' long enough, it gives me 1-2 seconds of normal operating sound, then powers down to the low hum and no cooking again. Is it over heating from the prolonged use? Is that normal?