Like mentioned above... I'd guess fan switch or air bubble.

Probably not the thermostat because it's not likely to get damaged or disconnected when you replace teh radiator. The other 2 causes are.

To bleed the cooling system.... remove the radiator cap (when the engine is cool). Start the engine and let it get up ot nearly full operating temperature (thermostate opens) rev the engine to 5-10 seconds at a time or until you stop seeing bubble come up. Add coolant as needed to top it off when your done.