I have a problem with my Encore hydro 48" not having the power to go up a hill. It has just started doing this in the last week. I have replace the hydro filter and the fluid level is good, repalced the drive belt, and made sure the cold weather start is adjusted properly. Still the mower is slow up hills. It works great and mows fast on flat ground. Any ideas on what else to check? The mower is four years old.