Rough idling

My 1999 Bonneville idles rough It seems alright when I first start the car and it is cold, but as soon as it warms up it idles rough. When the car is running and in park and the A/C running, if I turn the A/C off I can see the tach needle start to bounce up and down and then the car shuts off. While driving, if I come to a stop and the car is in drive I can see the tach needle bouncing up and down