We were camping last weekend and went to make some popcorn. The inside of the microwave started flashing, buzzing,smoking, basically holy smokes. Anyway I started thinking maybe now is replacement time. But the fix it side of me said whoa nellie,what could have gone wrong. So I did a little internet microwave detective work and found out there is a cover inside the microwave. This cover is made if mica and some other substance and somehow helps control the wave patterns etc. But if it gets dirty with food splatter or some grime a small hole can be burned through allowing microwaves gone wild. Well I gave mine a good cleaning and sure nuff, I had a black spot on my cover. So a little more internetting and walah! I found a replacement cover $2.50 plus 6.95 shipping.
It arrived today and I just put it in All Fixed!! No more flashing, buzzing,or sparks.
Had this happen on the home mw, bought the new shield material at a local appliance repair supplier but no joy. Clean frequently or after messy use.