Wow that's strange, I had the same think happen to my Hi-Tec rudder servo on my Flash this past Sunday, I was thinking that I should replace it
During normal slow movements of the stick as would be normal during flight they have little or no bounce and jitter around neutral. But if allow the stick to center rapidly i.e let go of the stick they both have quite a bit of bounce and overshoot of neutral. It dampens out in maybe one to two tenths of a second.