Microwave oven heats sporatically.

Question

I have a over the stove Frigidaire microwave (model FMV156DCA) that will heat normally for several cycles, then then next time i go to use it, it will not heat.  For a few weeks i was able to unplug the unit for 30 seconds, and it would then work again.  But now that is not even working.  I checked continuity on the door switches and they seem to be normal.  Anything else to check?