Servos are powered by the ESC's output to the receiver. If you're using a higher-strength servo than the ESC can support (usually greater than 20kg), 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.