Might be the contacts where the faceplate mate up to the unit are corroded enough that it's not making contact on one or two pins. You might try using a pencil eraser to clean up the contacts on both the unit and faceplate. Beyond that, the pounding from being on a boat might have taken its toll.
I run a round Jensen on my boat in the old speedometer hole. Not a high end unit for certain, but I love it so far.
If you're not living on the edge, you're taking up too much space!