It sounds like it could be an issue with the hydraulic fluid. Have you checked if it's at the correct level and if it needs to be changed?
I had a similar issue with my CAT 287C, and it turned out to be a clogged hydraulic filter. It's worth checking that.
Have you considered recalibrating the joystick with someone who has experience in this? Sometimes it's a calibration error that causes these problems.
Check the seals and cylinders, as any minor leak can cause this. I had a minor leak and fixing it resolved the jerky movements.