And the other common cause of unwanted movements is insufficient power...like trying to power servos from the Arduino instead of giving them separate power.
After reading twice and throwing away the first one; the base servo moves independently but the shoulder servo only jitters and freezes when I move it's corresponding potentiometer.
On the ATmega processors this is never an issue if the pots are 20k or less in value.
Sometimes there are issues with poor layout of the wiring to the pots - the 5V and GND
connections to the pots should not share any wiring with high current paths, this will put
unwanted signals onto the pots. Basically run the pot wiring as a separate harness from any
power or motor/servo wiring.