Toyota Repair: 93 Toyota Corolla Heater Problem, heater control valve, toyota corolla

Question
I purchased a 93 Toyota Corolla last Feb '04 and was told at that time that the heater didn't work well, so I had the thermostat replaced.  I've seen no evidence of leakage from the heater core inside the car on the passenger side.  When the temperature outside is in the 20's the heat is sufficient, but only when the car is running down the road....it blows cold when it is idling.  Also, if it's colder...I freeze in there....the heater blows cooler.  Any idea to the cause of this?  Is there a possiblity that I should have the radiator flushed?  Any ideas on this would be greatly appreciated, as I spend a lot of time in my car, as I sell insurance.  Thanks....Wanda Ross

Answer
Have the heater control valve checked it may be sticking partially closed, there is also a good possibility that the heater core may be clogged up, or a control cable from the dash control has come off at the air duct control unit under the dash which means that the heater is getting hot but the hot air is not directed into the car, try to have a toyota dealer check it out for those possibilities.
Since there is sufficient heat when driving at higher speeds I would be inclined to suspect the heater core.