Our 5 year old Kenmore microwave has run perfectly with no abuse, no
metal used while cooking, etc.
We have also used it to warm our plates, and typically we warm one,
two, or three dinner plates for between 1 and 2 minutes total. They
always emerge nicely warmed, not hot but just right. The plates are
similar to what we'd call stoneware but not as dense, perhaps some sort
of clay substance made by Pfaltzgraff. This shows the exact plate but
not the material, at http://www.pfaltzgraff.com/ca/13937.htm .

We have warmed plates in it for most of the 5 years since we discovered
this seems to work quite well. Today, while warming two plates, we got
a strong arc after only a second or two, which upon subsequent test
seems to occur behind the white plastic square piece on the right side
of the cavity (assume this is the magnetron horn?)

We shut it off immediately and then experimented further; it appears to
still function, warming a glass of water normally, warming a container
of rice normally, etc. However, placing the plates back in causes an
almost immediate arc.

So, what's changed? Have the plates somehow become more/less load now
(unlikely; we tried two other plates as well with the same result.)

I'm guessing maybe a buildup of grease or residue in the magnetron area
may have changed it's parameters somehow, to the extent that it's no
longer 'happy' being asked to warm plates.

Ideas? (Other than "don't do that" please...)