It took a long time to determine the problem but eventually I discovered that the plastic bracket (latch hook) that held the three door-switches showed appreciable wear from repeated door openings and closings. This wear stopped the door from sealing properly and the door switch safety mechanism blew the line fuse. I replaced the worn bracket (AP6230612) as well as the plastic latch mechanism (AP4553585) on the door that engages this bracket (and of course the fuse too) and the microwave now works properly.