I have a 2000 Mustang GT. At Idle when at a stoplight, with my foot on the brake, the engine will start to surge. As I sit there it will continue to surge worse and worse, with the tachometer needle jumping up to 1500 plus RPM until I have to put the car in nuetral to keep it from getting away from me. It is worse with the airconditioning on. It is bad enough that people will look at me like I am doing it on purpose. Any ideas? Thanks.