Cold air from Heater at idle and in lower speeds

check your blend door operation

Does the engine overheat at all during extended idle?? If so possible worn H2O pump impeller. Also partially restricted heater core can cause this. Make sure it is FULL of coolant.

May be pocket(s) of air in the coolant system. If the coolant has not been replaced in awhile, try having it flushed and replaced.