Basic systems engineering 101- when you have a change in the resistance in a wire in a signal circuit ( ie the connections in your servo extension) you will get a reflected signal pulse back. This feedback is causing the servo to overshoot. The greater the resistance change the larger the reflected pulse. Spray your connections with tv tuner cleaner and plug and unplug the connection several times and the problem will most likely go away. If it doesn't check the extension and or servo lead for a loose/broken connection most likely in the plug. Avoid burying extensions in aircraft where you can't get to them to plug/unplug them every now and then as they "clean" themselves when they are plugged/unplugged. I would recommend against the use of extensions and have extended/soldered all my leads for the last 15 years without any problems.