Sometimes when I'm trolling around my fish finder suddenly jumps to extreme depths and says it's anywhere from 700-1100 ft. deep and shows fish all along the top of the graph, which I'm guessing is actually the bottom.  My transducer is mounted on my trolling motor.  I don't have the manual for the fish finder, as it was put on by a previous owner, so I don't really have anyway of looking it up.  It's a Garmin.  I have no idea what the sensitivity is set for or really what any of that means.  Is there a setting that I could adjust that would help this, or maybe cleaning the transducer with something?  If so what is the setting and which way should I adjust it?  Also, what do you clean your transducer with if at all?  Thanks for any help.

Have you tried Garmin's website? They'll probably have the manual for your unit online there.

I have the same thing with my Eagle. As I recall it happens when I hve it set on automatic(seldom) and run into some shallow water with a very soft bottom. I just shut mine off and restart.

The suggestion to go to Garmin's web page and try to pick up and instruction book is a very good first step.

Maybe there is a secret underwater cave below the bottom of the lake stretching to 1000 feet below the surface in which prehistoric animals roam?!?

On my finder (a Humminbird WideOptic) it happens when I exceed 10mph. I think that the transducer, which is mounted on a hinge, gets pushed up by the force of the water. I also loose all depiction of the bottom while this is going on. When I drop back below 10 mph everything returns to normal.

You need to keep the transducer clean down to original plastic. I use Rain-X which both cleans and keeps it clean. Keep it tight so it won't move around. Also, in shallow water run the sensitivity way down, back up for deep water. I use a low ping rate in shallow water, higher for deep water. If you have noise rejection try turning it off or adjusting to any water clarity conditionws it lists.

Jim