The most likely cause of what you describe would be a radiator cooling fan problem. Your car has an electric cooling fan that pulls air through the radiator when the engine becomes hot; if it is not operating as designed then the car will overheat, especially at low speeds or when sitting still with no airflow from the vehicle movement going through the radiator core.

The reason that the car cools down with the A/C turned on is that most vehicles are programmed to command cooling fan operation whenever the AC is operating. I bet if you stand out in front of the car, you will not hear the electric cooling fan turn on and begin to blow air untill the AC is turned on.

This could be due to a faulty relay, poor electrical connection somewhere in the system, an engine coolant temperature sensor out of calibration, or an internal computer fault. I would recommend taking teh vehicle in to whatever repair shop you normally deal with to have the cooling fan control system diagnosed. This is not a very easy thing to do at home, unless you have a wiringf schematic, scan tool, DVOM, and are familiar with electronic troubleshooting procedures. The good news is that usually, with the proper test equipment available, diagnosis of this type of problem is pretty quick and easy, and therefore inexpensive.

Unless youa re driving around with the AC on all the time, it would be a good idea to have it looked at sooner rather than later. These engines are very sensitive to overheating, and can experience major internal damage (such as cylinder head cracks) from even one overheating episode.

I hope this is helpful; if so, an accept would be most aprpeciated! thanks!