Check the safety interlock (door) switches.. You'll probably find one that's partially melted.. (Which is typical time/use failure, since many uWave designs often run >10+ AC amps thru these little micro switches).
When these safety switches fail it often produces symptoms exactly like what you are describing.
Its important to remember that 2 of the interlock switches break the circuit when the door opens - if one welds, there is still a disconnect.
The 3rd switch is the vital fail-safe, that shorts the incoming mains in case *BOTH* break switches have welded.
If you're going to all the trouble of replacing and correctly adjusting the interlock switches - you might as well replace all 3 with good quality *NEW* ones.
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