Okay, here's my stab.
When you switch on the system, your servo is trying to centre itself to your radio's signal (assuming 1500uf) but as the servo steps only at 1502uf and 1498uf, it'll constantly "hunts" from 1502uf to 1498uf.
When you touch the transmtter's antenna, that could have shifted the signal to 1502uf or 1498uf.
Most digital servos have this "problem" due to their super precise deadband and pple think they faulty when they're not.
I bought a whole bunch of Hitec Digital servos because this is the only company with a servo programmer available to consumers.
Now, I am able to define my servos centre, absolute left/right end points, deadband and a whole lot of other parameters not available on the transmitter.
In addition, I'm also able to see the transmitter's signal as received by the receiver as well as the actual BEC output.
...opps ! sorry....beginning to sound like a Hitec advert !