Do you have a tach? If so what are RPM's light load and what with heavy load. My first guess would be it is over prop'ed for carrying heavy load. I'd drop 1 if not 2 sizes in pitch for those days you know your going to be loaded heavy. Dosnt take 10 minutes to swap prop. Find someone if you can that has a smaller prop and try it and see what it does. What size prop do you have.