I believe there is a shock attached to the speed control lever. If it is bad I know the speed will be erratic. Also make sure the linkage is tight and there is no play were it hooks to the trans. I have seen the hole in the lever were it attaches to the trans worn out.
oldnslo. you're probably younger and faster than I. Up and down hills requires constant travel speed adjustments. Some friction in lever. Force to move lever seems same as it's been for some time. Several years ago lever had more resistance.
JD420. Thank you for the insight on the linkage. I will get my nose into it at my first opportunity. Repowered 400 with B&S Vanguard last year. Getting hydro back to good will make for a honey of a tractor/