The speed controller provides power to the reciever for the servos. You do not need a separate battery. If you have simply plugged the connector from the speed controller into the reciever, as well as plugged a second battery into the reciever, that might be causing your problem. The output voltage from the ESC to the reciever is probably slightly lower than the voltage being provided by the second battery, which may be playing heck with your signal to the ESC. Try it without the second battery.

Brad