If you have a viscous fan hub The one where the fan spins free on the water pump then the symptom you describe is a result of it not working any more. With the engine cold start the engine and run at around 2000rpm If the fan is working properly you will hear the air noise from the fan for a few seconds then the noise will dissapear as the fan slows down. When the engine is hot viscous fluid ( silicon ) is pumped from one side in the fan hub to the other creating drag which makes the fan spin and draw air through the radiator. If this is not happening then the fan is stalled by the load of trying to move air and as such the temp rises. As you move off the air flows through the radiator and the fan becomes useless until you stop again. Noemal fix is a new viscous hub. If you have electric fans check that they are working as planned