just about all frsky rx's will operate on a 2s as will that servo, try bypassing the speed controller bec and powering straight from battery. remove +v lead from esc to rx and connect power to rx from battery. just check the spec of your rx first.
Alternatively you could try putting one of these in
http://www.hobbyking.com/hobbyking/s...arch=capacitor
sounds like your voltage isn't stable.
It might be the servo, presuming rx battery is nimh it will give more current then esc bec.