There are a few possibilities;
The system still has a pressure leak. The way to test this is to get a coolant sytem pressure tester. You can get one on a loaner program at either Advance Auto or Auto Zone. Test the sytem to see if it holds the specified pressure which is the pressure the radiator cap is rated at.
The radiator may be very corroded and cannot cool the engine sufficiently. It would need to be replaced and you would see corrosion down the filler neck into the radiator.
The radiator cap is bad and will not hold the required pressure for the system to work. Cooling system do require pressure to work and you can check this by grabbing the upper radiator hose the next time you turn off your car after driving and see if it has a resistance or you can feel pressure in the hose.
There may be air in the system. I will provide you with a walk through on bleeding coolant systems.
The temperature sensor could be wrong.
The fan(s) may not be working. They may need new relays or the temperature sensor for the fans, which is seperate from the temperature gauge sensor, has failed and the fans don't come on.
You can check this by getting an infra red temp gauge, less than $20 at Advance or Auto Zone, and point it at the thermostat and seee if it is actually overheating and see if the fans come on when they are supposed to.
The thermostat may be stuck shut and needs replacement.
Use the link below to help you diagnose and the other to bleed the air out of a cooling system.
Tuesday, May 15th, 2012 AT 8:15 PM