99mustang

I am in need of some help. My mustang is not running right. When ever i turn on the ac or the heat the engine idle goes nuts and some time just at idle with nothing running it just start reving. I mean it is reving to the point where i have to just about stand on the brakes to keep the car from jumping forward. anyone have any idea what i need to do to fix this problem? I have taken it to my local ford dealer and they tell me everything is fine, but something is going on. It is a 1999 Ford Mustang, nothing special, just a 4.6L V6. Any help you can give me would be greatly appriciated.