I had almost the same problem with a 9c on a 1/4 patty from world models. I tried four different brand of servos and extensions, moved the receiver and batteries, pluging into 2 and 8 reassigning 8 to another Chanel, using a y with a servo reverse, match box no match box, and every thing else that i could think of. The final solution for me was to shield each of the wires from the servos to a match box. ( I just used the plastic wire casing from an auto part store and wrapped a small copper wire around the out side of the plastic case and glued to one of the wood formers, the copper might or might not help against interference all I know is that it fixed my problem.)