When car is moving, you have air forced through the radiator. When standing still, at idle, ambient air is stagnant, and that's when you need the fan. It might be that you have a cooling system restriction somewhere, like an air bubble that prevents fluid from circulation properly. Or you have a problem with the thermostat. But you might have a fan that's not running well enough to cool the stopped car. There's a lot involved in the cooling system and you should have it checked to find the problem.
Also check the front of the radiator and air conditioning condenser for dirt and debris that can restrict the air flow at idle. It's definitely an airflow problem. Hope that helps! Jim
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