Bill,
If the unit is showing 12 to 786 feet deep it sounds like it is losing its track on the bottom. When it does this the display will stop scrolling until it again finds what it thinks is the bottom. Here are some things to try/check:
- Is there another sonar unit on? If so, turn its sonar (or the unit) off. Your Si unit may be getting sonar-to-sonar interference and can stop scrolling when the interference causes it to loose it track of the bottom.
- As Joby suggested: check the transducer connector to make sure that it is plugging all the way in. If needed take the power and transducer connector out of the mount/connector holder and plug them in by hand to see if this helps or not.
- Test your Si unit on another boat to see if this same thing happens and also test their unit on your boat. Connect only the power and transducer on each unit and there is no cause to be concerned about causing damage to either unit. This does not have to be the same model Si unit or even another Si unit as all you will be testing is the 200kHz 2D sonar here.