Problem with radio hit's
I would check a couple of things. First I would use a nylon clevis on the engine throttle. Any metal on metal vibration could be the cause of your problem. Second try your range check again with the engine running and the plane held or straped to the ground. see if you have any problems through the range of the throttle.
Your friend may be right. If you have to change your throtte setup it's better than picking up the pieces.