ok, first plug the batery pack straight in to the rx without the switch(old switches can develop dirty contacts in storage)then check operation.If there is still a problem plug the trottle servo into the rudder connection of the reciever and the rudder servo into the throttle connection of the reciever,if the problem travels with the servo to the throtle channel replace it,could be a bad gear or a dirty wiper on the feedback pot.also check for any metal to metal contact especially at the carb . post your findings.