After a nice lengthy summer with little use it might be an idea to give your microwave a quick check. By their very nature microwave ovens draw cool air in to cool the electronics inside. The cooling fan runs all the time the oven is cooking and draws air generally through a mesh at the rear of the microwave and then blows it through the microwave and out around the door. By design this prevents the microwave from overheating. Through the summer though with kitchen doors and windows open the oven will probably have collected some dust and fluff especially if you have pets.
The rear of your microwave should look like this picture above. You might even be able to see the cooling fan blade behind the mesh.
If looks like this picture below though, best give it a clean. Imagine how the flow of cool air is restricted. Its only a matter of time before the microwave will overheat and break down.
With the oven switched off you can use a stiff brush or even a vacuum to clean the vents. This picture was actually one that came into our workshop having overheated and cut out. If its really bad then maybe try to think to check it every few months just to be on the safe side.