My sounder did a very similar thing.

Prior to developing lines across the screen, a message appeared stating
that the unit was experiencing a low voltage condition and that further
operation might damage the sounder.

Immediately thereafter the lines began.

It still works, and I have a digital backup, but I'm hauling out the
first of the week for bottom paint, zinc, and a few minor repairs and
this would be an excellent time to install a new sounder.