Heater AOK but as I stop at a light it gets cold. Then goes ok on driving off

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In winter the heater only gives off heat if the car is driven. If I stop at a stop sign, it immediately goes cold and then goes back to heat once I start driving again. Had the rad flushed but did not help at all. Yes, liquid levels are good and the thermostat was replaced. Help please as winter is coming soon......í  Pierre