I recently started experiencing "bucking" on my 1997 520-H with the Eaton 1100 series transaxle. This occurred after 500 hours of heavy use with the 60" mowing deck, with all transaxle oil changes performed per specs. In reviewing causes of "bucking", I came across this for a similar complaint in a Simplicity 4711 that used the same transaxle. ( simpletractors.com/forums/topic/7441-simplicity-4211-hydro-bucking-problem/ ). One thought was that this was due to missing or loose bolts securing the transaxle to the frame.
This turned out to be precisely the reason for the "bucking" in mine. One securing bolt replaced, all other securing bolts tightened (along with the engine bolts), and the Forward-Reverse lever/linkage adjusted and tightened, and the tractor is back to "usual".
It's just something to consider before the nightmare of a transaxle overhaul...