cold air when heat is on

If it's cooling back off when the car is at a stop, this is telling me you have poor circulation in the cooling system. The coolant is not getting to the heater core unless the RPM's are up. You either have poor circulation in the cooling system or the level is low in your system. First you will want to check the level and make sure it's topped off with a 50/50 mix. If the level looks good, you may want to consider a cooling system flush.