why is exterior of the microwave oven getting hot & the lights getting switched off cooking pizza @ 180 degrees celcius then the exterior gets too hot and the lights on the microwave oven get switched off. after it cools down the lights start again

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The exterior is getting hot because it is generating a lot of heat.  A pizza has a bit of mass and 180 degrees Celsius is hot.  It could be a safety shut off.  

And it could be broken.  

If it works fine with smaller items then more likely it is a safety shut off.