How are the servos powered?
What type of servos are they? How many servos?
If you use just one servo does it jitter?
Servo jitter can be caused by a number of things. I think the most common cause of jitter is not enough power. Sometimes a capacitor near the servo power supply will help with jitter. Having the servo lead loop around a ferrite ring can also help reduce jitter. Some airplanes include ferrite rings on long servo leads.
Another cause of servo jitter is worn out potentiometers. Small movements around the same position slowly wears away the conductive film in the potentiometer. Once this sort of jitter starts, it quickly gets worse since the jittering makes the film wear out even faster. Some servos have replaceable potentiometers but this isn’t very common.
My first guess would be an inadequate power supply is causing your problems.