Jittery? A classic symptom of marginal power supply.
One way to get smoother action from the arm is to use a fairly slow speed. Also using properly sized and installed springs, to dampen and cushion the motion.
First, check all connections, and ensure the wires aren’t flexing too much. Wire is flexible but the strands will eventually part, with the motion producing spotty connections hence the ‘shakes’.
Next, slow the speed down to 25 and see if moving one servo at a time makes it go smoother. If it shivers while several servos are moving you should consider a beefier power supply.
Check the physical alignment of the joints, and tighten or adjust as needed.
My arm is kit-only, no mods yet, and it seems to work very smoothly with RIOS, at least at less-than-75 speeds.