Have you tried some bleach in there yet? Just a thought.

I had a HORRIBLE smell in my pantry when we first moved into this house. MIL and I peeled off all the contact paper, we bleached, we used some stuff to clean the toilet water tank in the camper, we used ammonia, we painted, and the smell was STILL there!! Finally, I just put in a bunch of those stick-up air fresheners. Couldn't kill the odor so I covered it up. It has faded alot over time (I suspect something had died in the wall or under the house right below the pantry). Not that it helps you any, can't put a stick up in your microwave (though if nothing else works, maybe you can put some air freshener thingee in there when NOT in use to cover it up).