Thank you for your question and I understand your concern. I do not have enough details in order to have a full and complete understanding of what you mean when that it stops and starts. If your implying the microwave unit starts up by itself without pushing any buttons then it could have key pad failure or computer board failure. If you're cooking on the range/cook and the temperature becomes too hot under the microwave/vent hood it is not abnormal for the fan to automatically come on and go back off once the sensor cools off.
Microwave ovens inherently store huge amounts of voltage even when unplugged which could cause severe personal injuries or could be fatal. Microwaves should only be serviced by trained and experienced technicians.