Please check your connectors. Lots of times, dirty connectors can cause the behavior you're seeing. If the oscillation stops when you have no load upon the servos, then you're OK. Digital servos will tend to "hunt" when they have little or no load upon them because the geartrain has to have a little bit of slop in it or it will wear prematurely. The tight deadband of the digital circuitry is what makes the servo try to find a center, and the little bit of slop in the geartrain prevents getting a solid center when there's little or no load upon the servo.

The air loads should quell the chattering you're seeing.