Is the belt new ? Or just good and tight. If the belt has hardened from use, it will have a tendency to want to slip on any pulley especially a smooth aluminum one that you have increased the rpms on by going smaller. Question though, How is going to an undersized water pump pulley supposed to help keep the temps down ? Unless you are running a direct drive fan without a clutch, the fan will still slip some at all speeds and the increased airflow if you get if any will only help below speeds of 35-40mph ( assuming you have the proper sized radiator fan shroud) after that, more air is forced through the radiator than the fan will pull. Speeding up the water pump can actually hurt the performance of the cooling system, the same way that taking the thermostat out will. The coolant has to be in the radiator long enough to lose enough heat to cool down the engine. Most guys will change the crankshaft pulley diameter to also speed up the alternator rpm's as the old charging systems were marginal at idle speeds.