BLKY2KZ

I have had me car for 3 1/2 years, and i am on my 4th alternator. The last 2 alternators i had on the car, everytime i would be sitting in traffic the volt guage will bounce back and forth between discharge and charge. While its doing this the lights will dim when its down in discharge. As soon as i start moving it stops doing it. It will not do this when the car is cold, only when its warmed up and only when im sitting at idle. The alternator i have on now is a brand new delco, not a remanufactured. Anyone have any ideas what could be wrong?