Sounds like your cooling fans on the radiator are not working. The fans are activated by a thermol switch on the engine or radiator reading the coolant temperature and an A/C switch. When the coolant is too hot the fans should activate cooling the system down. And, when the A/C is on the fans should also activate in order to cool the condensate so the A/C would work.
When you are driving, the movement of the vehicle throught he air is providing enough air flow to keep things cool and functional.
I would check the radiator fan fuse or fuses first. If those are OK I would try to run the fans with 12 volts and a ground that I supply just to see if the fan motor(s) actually run when powered. If the fan don't run when powered then the fans are bad and will need to be replaced. If the fans do run when you power them manually, then something is not allowing the power to activate, like a relay, control unit, or wiring.
It will take a little diagnostics, but it is not too difficult to chase it down.
SOUND LIKE LOW COOLANT LEVEL.NOT ENOUGH COOLANT IN THE SYSTEM YOU HAVE AIR IN COOLANT SYSTEM WHEN CAR OVERHEAT WHILE SITTING A FEW MINUTES. IF CAR START OVERHEATING WHILE SITTING IN A LONG TRAFFIC LINE.SOUND LIKE COOLANT TEMPERATURE SENSOR BAD NOT COMMANDING THE COOLING FANS TO TURN ON AT A SET TEMPERATURE.CHECK COOLANT FAN FUSE AND RELAYS MAKE SURE THEY ARE GOOD.HOT WIRE MAKE COOLANT FANS WORKING.IF NOT COOLING FAN NEED REPLACING.IF COOLANT FANS WORK OKAY.REPLACE THERMOSTAT AND RADIATOR PRESSURE CAP.YOU WILL BE ON THE SAFE SIDE TO KEEP ENGINE FROM OVER HEATING BLOWING A HEAD GASKET.BUT LIKE I SAY YOUR PROBLEM IS LOW COOLANT LEVEL OR COOLANT TEMPERATURE SENSOR NOT TURNING ON COOLING FANS IF CAR IS SETTING OR MOVING IN A SLOW TRAFFIC LINE.
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