My son was told the engine could have a cracked head as there was moisture inside of the filler cap when they changed the oil. My younger son thinks it has something to do with an ignition coil. This could be an engine problem, but it could also be caused by the automatic transmission. Moisture inside the filler cap is caused by condensation and is normal. The moisture would usually evaporate and be pulled out of the engine by the crankcase ventilation system, but the car has to be up to normal operating temperature for this to happen.
Lets look at possible engine problems first.
Spark plugs are the first item to look at when a car hesitates or bucks, but in your situation, this was only a temporary repair. A weak ignition coil could also cause a hesitation or misfire, but I would suspect it would occur at higher speeds. I think the problem may be fuel related. There are a couple of update programs for your fuel injection computer. The first update program fixed a hesitation problem caused by lean fuel mixtures.