All of my JR DS8411 servos do this after some use. You have to send them in for service. Mine have always been worn out feedback pots. The service folks say the gears are OK, but the pots wear out. I have had some servos in for service three times in 5 years. They always send back the old pots in the box with the repaired servos. As a diagnostic, I remove the servo from the plane and hook one large servo arm (no linkage). I have a homemade servo arm which is really a stick attached to a standard arm. With this setup, the servos that are bad will jiggle if the pots are worn. I do this to make sure it is not batteries, "Y" connectors, linkage, receiver, etc. The servos, even when bad, do not jiggle with just a standard servo arm on them. You must have a little mass - just a couple ounces - on the servo arm. This is enough to set off the oscillation when the pots are worn.