OK, looks like a mystery solved. We found a power lead to the KMA-24
that was chafing on the back of one of the other trays, and had worn far
enough to intermittently short, but apparently not long enough to pop
the breaker. It did it enough to leave carbon on the wire insulation at
the site of contact. That was repaired, and seems to have fixed the
problem. Another one for the books I guess. Thanks to all who chimed in.