I searched the web and it seemed that many people had a similar problem when gunk got on the waveguide plate/cover. When I removed the cover, one of the plastic latches broke so rather than just trying to clean the plate, I had to order a new one. The new one arrived in just a couple of days, I put the plate on and at least for the last few days, no more arcing. <$10 fix for my over the stove microwave.