Overheathing

More than likely it is the thermostat. It's probably the original.... Also check and make sure your fan clutch is engaging when the engine is at normal opperating temp. (you should hear a "roar" when acellerating in neutral. When you replace your thermostat make sure you propperly bleed your cooling system. There is an air bleed screw located at the upper water neck, and it doesn't hurt to jack up the front of your car so the radiator cap is the highest point in the system. Just keep adding coolant until you don't see air bubbles.....

-Steve