Plastic parts, is this a flotrol model? Not that it makes any difference, but its probably changing modes to shut off.

I would open the tank and watch it to see what its doing. It may take an adjustment, some lubrication, or the system might be hitting the top of the lid.

The adjustment might be needed to stop the fillup alittle short of water dumping over the overflow tube.

Hard to say, so many things could be the culprit.