The fan and radiator is doing its job cooling the system down before the car overheats. I would suspect coolant strength (get a $5 tester) first before trying the other more expensive fixes (thermostat, water pump, radiator).
Edit. It could be the Engine Coolant Temperature Sensor if all else seems fine. Check the wiring at the connector first.
Here is a post from Chongo. It highlights the ECT sensor.
Originally posted by chongo:
Did someone call me out.......LOL
here you go.