The 811 because it is a single ball bearing servo has a bit of play on the top gear. This play of the top gear combined with the servos aggressive centering abiltity has the tendency to overshoot the mark just a bit, and a slight oscillation occurs. If you can stop the jittering with light pressure there isn't anything wrong with the set up or the equipment, and it will not happen in the air due to flight loads. It would only happen on the ground. If however it takes more pressure, or does not stop with pressure, there may be a problem with the pot of the servo, or there is excessive slop, possibly in the gear train.
Also if you are using one, there is a chance that you may be having a problem with the 549 receiver as described here.
http://www.rcuniverse.com/forum/m_13...tm.htm#1350753