2004 Fastrak w/jerky hydro units

I'm using a Fastrak 52" while my Woods is int the shop. Pretty impressed with the mower. Its more stable on inclines and cuts faster & smoother. Thinking about replacing my Woods with it, but there seems to be an issue w/the hydro units. There is no smooth adjustment when compensating while mowing a hillside. The wheel on the uphill side seems to want to grab when you pull the stick back. Kinda have to zig zag your way across. The hydro gear units are older, and are sealed units, no filters. Just wondering if this sounds like an easily fixed problem, or a sign to stay away from this mower. The mower has about 585 hours on it, and doesn't make any funny noise. There seems to be plenty of power to pull straight up the hills, just is jerky when traversing or going down. I know the new Fastraks use ZT 3100 units, and weren't sure if older models could be fit with these, or if it was even economically feasible. Thanks for your input.