I've found these issues, and by eliminating them I no longer have problems.
When you switch on, leave teh Tx on for at least 10 seconds so it finds a good channel, before you switch on the Rx. Make sure that the Rx has a solid LED light before touching the sticks/wheel. This is because with all the WiFi and other 2.4 devices around, as well as our own radio, it can take a few seconds for the module to find a clear channel.
Make sure the aerial goes up, and is not flat for too much of its length. Make sure the aerial is not close (within 10mm) to any other wires, and certainly not touching them.
Take the case off the receiver and clean out any debris. This seems to be critical, as the voltage in the Spektrum units is low so any debris that gets on the boards can easily make a path that disturbs the circuits. In 12th, we also have rubber dust coated in oil that makes a nice little path! If you get dust and moisture together...
Make sure the PT is well away from the receiver and servo. We have known incidents where a PT on top of a servo, or next to a receiver, will give you what appears to be interference, but whatever it is, it makes the car do silly little things on its own!
If none of this works, then send both back to Horizon for a service - they are very helpful. HTH
