1999 Dodge Ram AC Blows warm when driving slow or stopped

I have a Ram 2500 24 valve turbo deisel and the AC works great and blows very cold, however sometimes when I stop in traffiic or traveling slow it blows warm air. It doesnt turn back to cold unless I turn the system off for approximately 2 minutes. Once I turn it back on it blows cold again. The problem is intermittent and does not happen every day.... just every once in a while but coming more of an issue. Any help would be appreciated.