Hi, first the gypsies warning, Whatever happens please remember you are messing around in a piece of equipment that runs at 4000volts and due to the design of its circuitry under certain fault conditions it remains lethal months after having been disconnected from the mains supply. its not rocket science as they say, but it is not for the layman either. Contact me via microwave-repairs.co.uk if you need further help.
ok now to the practical advice, I do not have my service manuals to hand right now, in fact due to its age we have probably destroyed the hard copies, however, if the problem is as you suggest, the remedy is to replace ALL the switches, 5 I believe in your machine. Watch out for the brittleness of the plastic switch holder, here may lie a problem, again I have a workround for that. The fact that you have to hold in the door button causes me a little concern about the door/switch setup.
kindest regads
patrick
August 2010