Why do my planes do this?
Suggest you get an assistant to help you do a range check with the engine running:
Agree on a set of handsignals your assitant can use to communicate to you. Have him/her firmly hold the plane either in the air or on the ground.
Collapse the Tx antenna, and start walking away, working the controls and watching your assitant. Check the controls at different throttle settings.
regards,
-Elliot