I want to play the guessing game. I have 3 rentals and one of them had alot of this. Believe it or not it was the breaker location. It was for the ac and they had both breakers on one pole and it needed one breaker on each pole to draw enough power. Not that this is what your problem is. Its just another option to try, move the breaker to a new pole. (if you have a 2 pole box) I could be way off on this.
I wanted to add breakers take a while to throw (a min to 2 is not uncommon). So depending on your exposure time the problem may have always been there. Add in the weakening of the breaker and you may have your answer. This is really all guessing, Just trying to help.
Whats the draw on the unit and what breaker are you using. It could just be a undersized breaker. (I know, you know what your doing but you have to ask.)
I found this on a forum:
the normal breaker size for wire size is,
#14 wire 15 amp breaker,
#12 wire 20 amp breaker,
#10 wire 30 amp breaker,
but should not be loaded for a continuous load of more the 80%
15 amp breaker and #14 wire, 12 amps,
20 amp breaker and #12 wire, 16 amps,
30 amp breaker and #10 wire, 24 amps,
If you have 18 amps on your unit you should be at 30 amp breaker and #10 wire, 24 amps