How can my overheating problem?

I have a 2005 iveco daily unijet 50c15, it has an over heating problem. At first the pipe going from the waterpump to thermostate housing had a split in it. I repaired it and now going up a hill even a small one the temperature goes through the roof, when I had the timing belt off if I didnt get it right would that make a difference, The motor runs great, starts great,had the radiator tested, that is ok, it is just going up hills.

Bruce
January 2009