Microwaves can be a big draw. If the voltage drops in the AC line the amperage(read heat) goes up...maybe enough to cook the supply.
Check the line voltage with, and without, the microwave running. Also find out what else is on the line that could be effecting the total draw.
Make sure the ground is properly installed on that line too.
If the line voltage is occasionally low with no apparent reason, check the earth ground for the building and the balance of two 220 volt lines in the breaker box.
