I wonder if Dale (digit) ever figured out what the problem was, considering this thread was from 2006.......

As for the poster who dug the post up, Ditch the flaperon mix to start with and see if the servo's do the same thing. I don't know Futaba stuff at all, but I did have a similar issue on a Kingcat stab when I switched to 8611's, turned out there it was the Y harness but as you say you're not using a Y harness I have no idea. Are you using extensions with amplifiers in them? That could be causing the problem if you're running 2.4.