Check this ouit first before you go out and buy a new mag:
The mag's little cooling fan probably broke loose and fell off the fan motor's shaft. The fan motor will still be running when you hit start but there is no fan on it. The fan keeps the mag cooler. The mag will heat up too much in a few minutes and then shortly the oven automatically shuts everything down when the mag gets too hot because the fan blade is just lying in there, Fan needs to be glued back on the shaft after the shaft is roughed up a little. Disconnect the plug-in wires (note where they go on), wiggle and remove the fan motor from the oven. I used contact cement to hold the fan to the shaft. the fan motor is located right behind the control panel. You have to first remove the grill, then the control panel's screws. It's not difficult to repair. No new parts needed. Poor engineering design of the cooling fan shaft. If the fan blade has not fallen off, try this: put some tin foil inside a baggy that is big enough to shield the control panel from the rest of the oven.