Castle ESC's have a notoriously weak BEC and the servo is drawing more current than the ESC can supply. Some radios like Traxxas and Spektrum are very sensitive to these voltage drops and glitches occur. The solution is either a glitchbuster cap (Spektrum or Novak sell them) which plugs into a spare receiver or an external BEC that bypasses the ESC for receiver power directly (funnily enough most people buy Castle BEC's to fix a Castle ESC problem!)

Either that or a servo that draws less power (Savox are amp hungry) or an ESC with a stronger BEC (Hobbywing SC8WP) will fix it.