[mikehinton]

A bad circuit breaker was my first thought, since the breaker was tripping with zero or minimal load on the circuit. I've already replaced the breaker, with no effect on the problem. The entire circuit is generally subjected to minimal loads - a couple of fluorescent lights (corded, that are plugged into outlets), and battery chargers. There is an air compressor that is used occasionally, but it's never tripped the breaker. Last night, after I unplugged all loads from the circuit, and reset the breaker, it tripped after 60-90 seconds. Very strange...

There are three other breakers in the sub panel. None of those has ever tripped.