The most seriously or dangerous reason for this to happen would be a faulty and leaking magnetron! I have seen them fracture one of the circumferential ferrite magnets and emit microwaves outside of there strutures, into the appliance casing. The microprocessor timer board then gets zapped with the microwaves and protests! You should have it proffesionally checked.
Although of course it could be just a problematic connection which may be inexpensive to repair.
Microman
April 2009
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