Hi. I already had similar problems like this and, unfortunately, in all of the cases, the circuit that control the motors, was burned. The easiest way to solve the problem, in my case, was replace the mother board . I've burned out two RX-14 and two RX-18. All of these boards had the same circuit ruined.
You can find the board to be replace yours at www.rscalemodel.com
Thanks,
Jeferson