Have you tried mounting the transducer on the TM shaft? I had mine mounted on the barrel (port side) at first, It did pretty good. I used the rubber pad they sent with it in between mount and barrel. I since have moved it up 12 inches and mounted port side on shaft and I'm getting better returns. Wouldn't hurt to try only takes a few minutes to move it, I would do that first before calling Garmin.
Also, do you have the prong grounded on the unit, and using the magnet clamps on the cables? I don't use either, but just throwing out all ideas.