I've had this exact problem before. The servo is drawing lots of power and the ESC can't supply enough current for both at the same time. To solve this issue you can either buy a "glitch buster" or an external BEC. Novak and Spektrum make glitch busters. An external BEC, like the 10 amp one from castle creations is a more secure way to eliminate the glitching as it takes power directly from the battery to the receiver.