the condition you describe is what happens when the radiator cooling fan is not working. the car won't overheat immediately because you have to drive it for a few miles to get the engine up to temperature. when you're driving there is enough air coming over the radiator so that the heat is carried away. the radiator fan is only used to pull air over the radiator when you're going under 30 mph or you are at stops. if the temp rises as you slow down and come to a stop then the radiator fan isn't working.If you have air conditioning on the car then you can turn it on when you see the temp rise; if the temp drops with the air conditioning on then you've confirmed that the radiator fan is inoperative. turning the air conditioning on turns on the condensor fan automatically and that will do what the radiator fan normally does.the usual cause of a radiator fan not working is a seized fan motor. there is a temperature switch that turns the fan on that can go bad but typically its the fan motor itself.hope that helps