Simple question,

What would cause an engine to surge while stopped at idle. For example : car comes to a rolling stop at a traffic light, while foot is on the brake waiting for the light to change (in Drive) - car suddenly surges and slightly lunges forward for a moment. Almost immediately after this event, the idle returns to a steady "normal" state. To describe the sensation would be as if one depressed the throttle momentarily and held the brake at the same time. In the course of several dozen stops and starts in normal day of city travel periods, this only happens once in a while and is not consistent with any other condition.

My best guess is that there is some form of back pressure "pushing" the throttle plate open and triggering the TPS. but I have no way of knowing this while driving.
Any other ideas that may cause this symptom?
Please no replies containing MAP sensor , as the Regina system does not use one.
Thanks.