I heard about this problem with the old forest river camper we had. There is a thermal overload device located inside the unit. The ones I have seen are called klixon's. They are automatic resetable thermal overloads. (normally located on the High voltage control side [your clock will still work])
The reason for the shutdowns were that the cabinet they were mounted in did not allow enough air circulation for the magnetron.
Here's a quick thing to try. Put a small fan outside the micro to see if you can increase the air flow around the unit. Some people were removing the microwave and it would work fine on the bench but not in the cabinet.
Here is the link for the Klixon if that is the problem. You can test it with a multimeter and a hair dryer. It may look similar to the top three on the left.
http://www.sensata.com/products/cont...therm-6786.htm
You have to remove the outer cover of the microwave to find this device.
Do not run the microwave with this cover removed. There are potentially hazzardous voltages located within. Standard disclaimer about qualified personnel.