Yep, Ferrites near the RX.
Saying that, I never used any, and God, some of my gliders have meters of cables!
But rule number one, use twisted cables. Always!
I believe the twisted cable "shield" itself, but anyway, whatever the sorcery behind it, no ferrites in my case, never had a glitch (in 41 or 35Mhz).
This might not be the case with 2.4Ghz, as of not needed anymore, but in this case, I would maybe suspect a low voltage on the RX output (loss of volts because of the long cable maybe?)
Have you tried different servos on the leads?