ORIGINAL: Live Wire
Your battery at full charge, being 6v could cause this unti it is on for a while. Then the Tx to close to the RX can do thisl. try a range check and see what it does.
This is your answer right here. This is very common with fully charged batteries and lower level servos. The other thing that concerns me is how far out you have your linkage on the servo. Not sure but the airplane looks to be some sort of warbird and does not need that kind of throw. Check your dual rates and ATV, if either is dialed down less then 75% then move the linkage in on the servo. This will further help your situation.