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Can be part of the power supply or the controller board. Best to take the unit in for service. You would need the spare parts to try, the service manual, and the proper voltever to make the necessary verifications.
Most of the times the intermittent problem is due to the door safety microswitch. You will have to take the cover apart and check the two or three microswitches that are actuated in the close/open action of the door. They can be located very close to the locking levers on the right side of the window. TO check these Microswitches I suggest to use a series circuit with an ordinary filament lamp and a suitable battery or Power supply. These microswitches have three terminals, although, maybe not all 3 used, They are: a Common, a Normally Open and a Normally Closed. Please take care not to missconnect or change the terminal positions. A good practice is to take a picture of the connections with your cellphone prior to disconnect. Good Luck.
I'm fairly sure the fan has a door switch and that may have failed. if you take the outer cover off the micro wave itself you hopefully will have a wiring diagram of the unit. Many times I have had to replace micro wave door switch's that have failed not letting it start or sometimes such as yours something like the light staying on. Being that you are a service man, I don't have to tell you be sure to disconnect the power to it and then you can use your ohm meter to check the switches. Hope this helps you
Did you disconnect and test the three latch assembly microswitches with a multimeter or continuity tester? The top and bottom switch are normally-open, and the middle switch normally-closed. The lower switch seems to be the one that takes a beating and most often fails. It should be an Omron D3V-16G-3C25 and can be found on eBay for $10, or via an electronic parts supplier.
Poor solder joints are a common source of trouble on printed circuit boards. You might want to remove the control board and check for poor soldering (grainy joints, cracked solder, occasionally even unsoldered) or cables that aren't seated properly. Also, electrolytic capacitors are a frequent problem in electronic circuits. These are supposed to help filter and smooth out voltage and if they don't do their job you can have unpredictable problems. Capacitors are cylindrical parts around 1/2 to 3/4 of an inch high and around 1/4 to 3/8 in diameter. Look for swollen parts, bulging or split tops (they should be flat). If there are no visible problems the board may have a flaky microcomputer, possibly (as you've mentioned) from power transients. But bad connections are such a frequent thing with current equipment that I'd look for those first.
These machines seem prone to having a bad magnetron. Look for a circuit breaker near the power cord entry point. If tripped, reset and try to heat a cup of water. If it makes a loud buzz and then pops the breaker again, the consensus view is a bad magnetron. This is one repair that is recommended to send out. (~$150). You need to evaluate the probable cost vs a new machine.
This was my first microwave oven repair, and I feared the worst - big $$$'s, parts no longer available, etc. But after I pulled the unit forward and found the schematic diagram, it turned out to be a defective microswitch in the secondary door latch circuit.
Even in retirement, my degrees in electronics and computer science occasionally save me some $$$'s.