That looks like a classic bad pot to me. If you'll spend a little more money you can have a good performing servo. Don't expect any of them to last long, especially if you're doing 3D flight with them. For low cost but serviceable servos, look at the Solar line. The one complaint I have is that the wires going into the motor and board don't have any vibration protection, but that's easily fixed with a dab of hot glue. Solars center well and are accurate. For quality and durable servos, look at Hitecs or Futabas.