I think I know your problem by my experience I may be wrong if so someone correct me please? But from memory some digital servos dont like fm/am - rx/tx systems.

So these are the things I would try first to fix it.

1st- if you have a friend that has a 2.4gz radio system (set this up n see if this works) if no friend look on ebay there are some ok cheap ones there (before buying one check reviews).

2nd- you could try a cheap analog servo to suit your current radio system


hope this helps
Ray