DIAGNOSIS This is a thermal cutoff. The unit is overheating and shutting down to protect itself. The most common cause is a blocked or failed ventilation system.
FIX Unplug the microwave. Pull it away from the wall. Vacuum the rear grill and the top vent (if over-the-range) thoroughly. Check that the internal magnetron cooling fan is running when the unit is on. If the fan is not spinning, it or its motor needs replacement.
IF THAT DIDN'T WORK The thermal cut-off fuse or the magnetron thermal protector may be failing intermittently. These are located on the magnetron or air duct and require a multimeter to test when hot.
HEADS UP Operating with blocked vents can permanently damage the magnetron. Ensure at least 3 inches of clearance on all sides, especially the top and rear.