JD 4320 hydrostatic transmission surging

[remjr195611]

My 4320 transmission is surging up and down. When a greater load is put on the hydraulics either by driving up a hill or lifting my bucket the engine revs up and down. I changed the Tansmision oil and filter. I still have the same problem

 

[Katahdin]

I suspect its your fuel and/or air filters that are dirty.