Microwave re-started

Question

I have a 1991 Panasonic NN-5560A microwave oven that re-started on it's own. I was heating a frozen dinner on medium power for 8 minutes. I left the room while it cooked and heard the five beeps indicating it stopped cooking. After a couple minutes I entered the kitchen to find it had re-started and the clock said 6 minutes left to cook, still on 50% power. My dinner was overcooked with the extra couple minutes, so I know it had completed the previous cycle. Have you heard of this happening and is it time to get a new microwave? We've never had problems with it. How would it re-set itself that way and turn on again?