Just recently with all the hot weather, we have had several ovens come into the workshop with various symptoms and faults but all with the same cause. The oven had overheated because the air vents were blocked.
- this air vent needs cleaning !
Microwave Ovens rely heavily on cool air being drawn in (usually through a vent at the back) which cools the components inside. Usually the oven isnt on for too long so it doesnt get that hot but with the warmer ambient temperatures we are experiencing it doesnt take much to raise the temperature. Most microwave ovens will have a vent or area of holes on the rear.
This is where it dras in the air and because of this over a period of time will get covered and eventually blocked with ‘fluff’. It can completely block the vent and will, if not cleaned off , cause the oven to overheat and break down.
Microwave Service Company Ltd 
Is it safe if a microwaves back vents are facing you. I sit in a office where a that is the case, I can feel the hot air from.