Have you tried tapping on the receiver to see if the controls do strange things from the vibration?
Also check to see if the transmitter antenna has become loose or unscrewed.

Get someone to help, and do a good range check on the radio - see how far away you can get and still have control, with and without the motor running.

You may also have a problem with the battery charger.

If you do have to send it in for repair, it would probably be best to send it all so they can check it out - transmitter, receiver, batteries, battery charger, etc. Electronics can fail suddenly, or even develop a crack in the solder connections (resoldered lots of those in my former job working on audio equipment).