By "output" are you referring to values, as in position, or pin numbers, as in the servo attached to pin N jitters while the servo attached to pin M does not?
The vast majority of servo issues reported here turn out to be insufficient power. Having an external power supply is a good start, how much current can it provide? Also, what voltage is it actually providing when the servos are powered up?
Servo attached to pin 2 and 3 doesn't jitter, attached to pin 7 or 8 jitters.
If you have servos attached to pins 2, 3, 7, and 8, and you move the control wires around, does the jittering follow the pin? That is, if you have a servo connected to pin 2 that does not jitter, and you move it to pin 7, and move that servo to pin 2, which servo now jitters?