Kenmore microwave shuts off after several seconds

Question

Problem is two fold. Our Kenmore microwave model # 721.80592 fan started running for extended periods, when not in use. Problem seemed to arise when water was boiled on stove below. Initially this seemed to be solved by pulling plug. Problem has now morphed into microwave shutting off after several seconds. Thermostat was replaced with no effect. Your help is greatly appreciated.

TS