Check your fuel pump output volume and pressure. Might be the pump, might be the filter or maybe you didn't open the tank valve all the way when you last changed the pump.
With restricted flow, as you get toward the top of the hill, the float bowl gets lower and the mixture gets leaner and the power drops. Yet, unless it's real bad the engine doesn't missfire. It can fool you. Most re-pop pumps aren't very good. I went to an electric pump and it solved a multitude of nettlesome little problems.
