Driving a new GT and noticing what I call an intermittent engine surge or idle stumble. Only happens when the auto stop/start is engaged. It's hot here in TX and when the a/c is running and I stop, w/in a few seconds the engine rumbles and vibrates after it's been cut off automatically. When I shut off the auto stop/start, it doesn't do this. Do I need to go back to dealer to have this diagnosed? Thanks.