Our Microwave keeps running unless we give it a whack!

We have a Danby counter top microwave. The problem is this:
If you open the microwave door, and then close it, the microwave starts running - i.e. the light is on, the fan is going and the turntable is turning. If you give the microwave a whack on top (or just shut the door with a more sudden "snap") it will stop, and then you can set the time and hit "start". It's been doing this for a long time, and it seems to be getting harder to make it stop! We don't know if it's actually microwaving when it does this, or if it's just the light, turntable and fan that are running. Wierd, eh?