Microwave oven stops spontaneously

Hello,

My microwave oven with grill stops spontaneously when operating just as oven.

Sometimes it stops within 1 minute after turning it on, sometimes after approx. 10 minutes when baking a pizza (both time the timer was set to 15 minutes or so) and last week after approx. 35 minutes baking lasagna (timer was set to 50 minutes).

Any ideas what the problem could be?

I already checked the 3 micro switches for the door protection (and cleaned the contacts) they are working just fine.

Thanks.