Servos are powered by the ESC's output to the receiver. If you're using a higher-strength servo (usually greater than 20kg) than the ESC can support, you may experience 'brown-outs' where the radio signal can be interrupted and cause erratic driving. This can be helped by plugging a Glitch Buster into the receiver, smoothing the current delivery. However, in some cases, you may need to install an external BEC.