I had a similar issue on a new trailer, it would intermittently show a short when moving. Dealership could not find it, it was still under warranty. Every time they said it was fixed I would pick it up and short would show up on the way home. I isolated the problem by jacking the trailer up and spin one wheel at a time until the brake controller indicated a short. Brought it to the dealership and pointed to the wheel that was causing it, when they removed the drum the brake wire had come loose from the clip inside and was just barely rubbing on the inside of the drum. They fixed the wire and have not had any trouble since.
