Check the heater hoses to see if they are still hot when the air goes cold. If the hose are hot it is not a cooling system problem but a temperature blend door problem. If the hoses have a different feel to them,one hot and one cold or warm, it is a circulation problem and the coolant is not moving through the heater core fast enough to keep the air hot out of the vents. This usually ends up being a clogged heater core.