Why is my car over-heating?

Your thermostat may be stuck. Your mechanic will be able to tell, don't put off taking it in to be checked. A thermostat is not an expensive repair. When you are driving the wind tends to cool down the motor more while when you are going slowly, the engine over heats. Do not put off this repair no matter what, you could have your motor seize up on you and then you'd be in big trouble - $$$$$ or more likely an new car! Do not drive the car when the temperature is high.