ORIGINAL: BarracudaHockey
Weird. Amplified Y harnesses and that are no-no's on Spektrum system but your setup should work. My guess, since 2.4 really doesn't react to ''interference'' like that is that you have a vibration problem. The first thing I would try is reseating the connections in the extensions.
You also might try it with one at a time plugged in.
I have to agree with you.
Try changing the servos.
It is extremely unusual to have interference on one channel and not the others.