This is very common, here are some common causes and solutions:

On a new (or fairly new) 8411 servo:

1. Try tightening the cables. Often this is all that's needed to fix the problem. You should be aware that changes in temperature sometimes cause the oscillation due to shrinking/stretching of the fuse and cables in different temperatures. I've experienced this.
2. If #1 doesn't work then it's possible the dead band is too narrow. The "proper" way to fix this is to send the servo back to Horizon and have them increase the dead band. However, another option that will sometimes get around the problem is to use the sub trim to move the servo off of center and then replace the servo arm so that it is centered. For some reason changing the center will sometimes fix the problem. This is a good solution if you don't want to wait for a round trip to Horizon.

On an older servo (I'm assuming it's not always had the problem) you can also try tightening the cables a bit, it's possible they've stretched. If this does not fix the problem it's probably time to send the servo in for an overhaul. The off-center trick will also sometimes work on servos with worn pots, but I prefer to send them off and get them overhauled.

Remember that JR has a three year warranty and they will repair even worn servos free of charge if they are in warranty.

Keith B