First off, there is a tiny bit RF energy being emitted from the ignition system in ALL ignition systems. It can be seen on a sprectrum analyzer and this is normal. Most ignition systems keep this to a reasonable level that can be tolerated by most modern radio systems.
If there are no other symptoms, my guess is the problem might be the long leads to the rudder servos combined with the analog servos. Have you tried the digital equivalent or something better than the 225 servos you are using. Sometimes just this simple change will resolve a problem like this as most digital servos are more tolerant to the ignition environment. A different problem might be the Y connector. I have seen servos on Y cables act strange even without ignition noise being present. Any way you can run separate leads to the servos.