Then you'll have to define "surges". The engine turns the transmission, and the transmission moves the tractor. So if the engine stays at the same rpm, it's not an engine surge. The only other possibility IS the transmission. First recommendation is to check the fluid level. Alternatively, you didn't perchance top it up with the wrong fluid (?). It came from the factory with Lo-Viscosity HyGard, which is all you should use. Not necessarily the stuff from the dealer, but definitely something that meets/exceeds the JDM J20D spec. Regular HyGard and "universal" fluid is too thick. I get a Lo-Vis equivalent from Rural King (Harvest King J20D) for about $10 gallon.
//greg//