or if its the batteries which cannot provide enough amperage for the servos under heavy loads (the battery pack is 4 AS's).
Ooh, ooh, I know. It isn't the software.
Oddly enough it could well be the software
If the timing loop that is setting the time between the PWM is too long the servos stop trying to seek, they go into no resistance mode. If the servos are under a small load that causes them to move off the desired point slightly when this happens then they will indeed jitter or oscillate about the desired point.
But then a really
big battery might fix it too