why losing heated air from car blower?

My Chrysler Voyager 2002 with 110,000 miles blows warm/hot air from heater ONLY when car is in motion. If car is sitting idle at a light and heat is blowing, the air slowly cools. If I put car in neutral and increase idle speed, the heat starts to come back. Once I start driving again and drive off, the heat returns. Any ideas what is wrong??? Thanks