I would plug an old servo into the throttle channel and disconnect the ignition kill and see if the receiver is causing the problem. If you think all is well with the receiver, plug the cutoff back in and see if it burns it up again. Then it would be the kill switch. If it is burning the servo up with the throttle pushrod disconnected you might see if you take it out of HS what happens. I assume the pushrod is free to move with no binding.