What servo, what esc? Sounds like your servo is pulling the voltage down. Is it more powerful?
it could be your over driving it, if it's trying to go past the mechanical limit of the steering it will stall and pull excessive current, if it pulls more than your bec can handle then brownout and disconnect. check your travel and as mentioned fit a capacitor, I always had one on ectronix and spektrum.
http://www.hobbyking.com/hobbyking/s...voltage%20prot
It could also be the servo is either mech or electrically faulty.