A little confused about this servo behavior. My two new Futaba S3003s seem to give a short burst of normal function, then begin to shake and move slowly. Here's a video of the Arduino sample servo code, Sweep, on the servo in question: https://www.youtube.com/watch?v=xqNYS_C9TJA
Here's the same code, and same connection to the Arduino, with a (working) AR3606-HB continuous servo: https://www.youtube.com/watch?v=w9CKWj-W1XY
The only potential reason I can come up with to explain this behavior is that I immediately used an external battery pack with the two new servos. It's possible that either the ground or power cables weren't exposed enough from their shielding during first use. I had cut some shielding after trying both servos, but am getting the same results (did I damage the servos?). The external battery pack works well with the AR3606-HB. The pack holds 4 AA batteries, matching the required voltage for the Futabas.