Sundstrand Hydro. New oil and filter in the unit. Unit is full, free wheeling valve is closed. Going forward on a little pull it starts to jerk. Level and down grade it works fine. Can go in reverse full speed and it stays smooth as can be. Only in forward on a pull. I suspect a linkage problem some place but could not see any thing out of place or excess wear. Any suggestions as to where to start looking for the problem?