type of fluid is in the transmission? Hopefully super udt2. Anything else can cause chatter. THe clutch is hydraulic kinda like an automatic transmission in a car and improper fluid can cause the clutch pack to chatter. The bad part is, once the cheap fluid is impregnated into the clutch discs, there's no getting it out short of replacing the clutch pack.
Also could be an issue with the hydraulic system that engages the pack assembly.
Or-one other item, look under the tractor. There's supposed to be a spring between the "clutch" engagement mechanism and the tractor. If the spring is gone, they'll get real jerky on engagement.