Problems with 2721 servos !
I don't think you are having any radio-receiver problems. Is there any slop in the gears, i.e. worn gears, or any other slop or flexible pushrods? Also make sure that the servos are not mounted too rigidly, i.e. screws too tight, mounting rails too tight on the servos. Also if the servos are older and have seen a lot of use, the pots may be worn causing the problems. If you can touch the surface and it stops, this is not likely the cause though.