I've seen this happen on several Lowrance models (mine included). I currently have an inside the hull mount and it's hit or miss on how it will read. Most of the time the transducer stops picking up around 20-25mph (sometimes a little faster, sometimes slower), but sometimes it just wont work with much speed at all. Maybe Lowrance could offer a suggestion for you or there's a pretty good forum for Lowrance issues on BBC. I've learned to live with mine for now.

http://www.bbcboards.net/lowrance-garmin-sonar-gps/