I have a 9100s that was doing this in my 8ight-B. It was a new servo and I couldn't figure out what the deal was. If there was no pressure on the wheels it would jitter in a very annoying way. Everyone that saw me on pit row would ask or comment on it. Several people said it was common with digital servos and the 8ight, something about harmonic interference shaking the servo, and it's constantly trying to find center. I just ignored it since it didn't happen when the buggy was on the ground.
Well about two weeks ago I noticed that it wasn't turning smoothly to the right, it would pause about halfway to full throw. I figured the servo was finally done and took it out to replace it. After taking the servo out I noticed that the steering was still catching at one point and had tension through the whole range to the right, with no servo attached. I took the front end apart and found that the collar under the spring on the servo saver had pushed itself out from the stock dialing and that was causing a slight steering bind. I corrected that, put the 9100s back in and it worked perfectly. No jitter and no travel issues. I've run it for about 40 minutes since then without any problems.
Long story short.... check for something binding before writing it off. That slight tension on the steering was enough to cause the servo to fail to find center.