Microwave heats only for first 30 seconds of a cycle.

Maytag Microwave MMW5208WS-1 Microwave is no longer heating properly. I hooked it up to a device to monitor wattage and tested. After the unit is unplugged and plugged in again it will heat at approximately to 1980 watts for 30 seconds, after 30 seconds wattage immediately drops to approximately 140 and stays there for the rest of the cycle. The microwave still appears to be running normally lights, turntable and fan are operating. When attempting to heat for another cycle the wattage never changes from the 140. Unplugging it essentially resets it, but again only heats for 30 seconds of the runtime of the cycle the drops to 140 watts for the remainder of the cycle.

What are the possible culprits? Diode? Capacitor? Or something else?

TIA - Shannon