1. Check all your wires for bare spots.
2. Make sure that the antenna wire gets covered where it is bare, also check to be sure it isn't broken or cut short.
3. Route all your power wires away from your servo wires.
4. Make sure all connections are secure, and not damaged.
5. Make sure your batteries are fully charged, (receiver and transmitter).
6. Make sure the receiver and transmitter crystals are fully installed and not loose, or damaged.
7. Make sure all metal to metal contact points on the car are tight, so they will not vibrate (this can cause interference).
8. Can't think of anything else, but I'm sure others will be able to add to this list.