Getting any transom-mounted xducer to work reliably is all about proper placement on the hull. Is your boat a cut-out transom or is the OB on a transom bracket? Where is ther placement now? Where does the manual state to place it?
12mph would be where the boat is beginning to come up onto plane, so something, either from the hull or motor wash, is causing air bubbles under the xducer face. I'd have someone else drive it while you look carefully over the stern and see what kind of water the xducer is running in ... it should be in relatively clean stream of water exiting the hull. You also might not had it deep enoough or enough back tilt put in so the signal can bounce back to the face.
