That sounds like it could be a water pump propeller, but you wont know until you've done it. When your car is idling the temperature will fall back down to normal running temps, so the water pump wont be needed so much. But as soon as you get onto the motorway or put the car under any added stress, it can't cool it quickly enough so the temp rises.
It's worth changing the propeller regardless really, but requires the cambelt to be taken off so you may as well change it all in one go... cambelt, brass propeller and tensioner (if poss).