ORIGINAL: dirtybird
If it is a 2.4 system I have to disagree. 2.4GHZ is nearly immune to such interference. If it is a 72MHz system maybe so.
There has to be something wrong with that servo.
I've had ( Hitec ) servos hooked up to the throttle on a gasser react exactly the same way.
I found that the servo electronics were getting swamped by the RF from the ignition module.
I could reproduce the problem with the servo disconnected from the linkage... a replacement servo did exactly the same thing...
I switched to a different brand and the problem went away.
We also had the same thing occur, again using Hitec's, on one of three club trainers that used DLE 20 engines... switching to a different brand also cured the problem.