I've run servos in the stab on just about all my planes and never had range/interference problems. I use Futaba. On my big planes I do use twisted wiring that runs from the servos all the way to the receivers - again, never had any problems with that setup either.

As an experiment - try replacing one servo at a time to see if that clears up your problem. I mean ALL servos, not just the stab servos. If that doesn't work, then replace the RX (if you haven't done so already) and then lastly, try replacing the switch and battery pack. One change at a time and you may end up solving the issue.

-Derek