Then I would look into a leaking headgasket depending on how much water is to be refilled during every drive.
Because you mention you have to refill the radiator everytime, would tell me there's enough coolant leaking away somewhere to cause overheating the next you drive it.
That alone gives the impression the coolant system is very marginal in the first place.
How much water needs to be added everytime anyway?
Are there wet-spots under the car?
Does the oillevel rise, or look milky?
If your avatar image is still correct, your underdriven waterpump doesn't help the situation either if the coolant level is low. But this won't cause coolant to boil in such a short time.
