Not exactly like your problem, but I just had to replace the fan motor in our 15,000 BTU air conditioner (we have two), as result of low voltage in a park we were staying at in Colorado in August. I saw voltage on the meter as low as 96 VAC when the motor failed, and after that any time I turned it on, it immediately tripped it's 20 amp breaker.
Have you checked the voltage? If it is low, it will cause it to draw more current and trip the breaker. Anything like a over heated socket could cause it, and may well be your problem working on a 20 amp circuit.