The problem here is either going to be the magnetron, High voltage capacitor, High voltage diode or the door switch. This is not a good DIY fix because factory test equipment will be needed to accurately diagnose the problem plus the unit stores lethal voltage even when unplugged.I recommend you take the unit down and take it to a experienced workshop to have it fixed. The cost would be anywhere between $150 - $250, Bearing in mind that most new units cost around $229 you may want to look for a new unit.After the warning given if you decide you want to fix this yourself and have a multi meter and amp meter I would be happy to assist you further :)
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