2007 Chevy Uplander, While at idle the heater blows cold, but while vehicle in motion heater works properly.

The heat can be turned up all the way and while you sit in driveway or idle at a light or in traffic it blows cold, but while you are driving weather its 25 or 50 it blows hot. The coolant is new and full and I see no leaks from hoses or other areas but that dosent mean their not there. I was wondering if it was thermastat or waterpump. Maybe some other bigger problem? Any idea appreciated.