*scratches head*

What you are describing is RF interferance... if your battery gets low or is not holding a charge properly your servos are going to get weak. RF interference does not go up because your batteries aren't performing? /*scratches head*

You don't mention doing a proper range check... are you doing one with and without the engine running? You should be able to get close to or better to 90% of the range with the engine running that you got with it off.

Things to check


#1 Metal on metal contact anywhere on the plane - ie linkages etc
#2 Is your spark plug cap on tight and all the way?
#3 Are you using a resitor plug and do you have another one you can try.. bad plug can cause this
#4 Go back and look at all of your linkages again - especially throttle.. make sure plastic connections at both ends.