The best way to investigate this, is to eliminate each variable, set a new model No in the Tx and reset everything in that model memory so it is blank, use one battery plugged into the Rx direct and then attach the two servo's concerned into seperate channels, if it still happens you probably have a dirty pot in your Tx or servo, if it does not happen set Flapperons and try again, if it then happens you have a rouge set up on the Flapperon mix in the Tx, keep going until you have the problem solved or found and eliminated, its the only way to test each variable, unless you have another Tx you can try.

Best of luck
Mike