I can time an idle stumble every 30 seconds while at a stop. Auto tranny, in gear, at a full stop waiting to go. RPM drops from 800 to 300 every 30 seconds. Like it is re-setting itself! 2013 GT. bought used a few months ago. 55K miles. In really good shape. Anyone know what this is? Is it possible this car had a tune that is creating this situation?