I have just made a gear water pump for my Centaur Gas engine, I used Hostaform gears (Delrin) the pump works good until the water flow gets warm and the belt drive starts slipping I put this down to a problem with the belt itself.
But reading Art's suggestion that it could be a expansion problem with the gear material when warm, I looked up Hostaform Gears and found they have a Maximum operating temp of 90 degrees Centigrade, now I don't know if my engine gets to this temp but I think Art may well be on the right track.
I think I will try a change of gears to either Brass or Stainless.
Mike.