Casio CZ/ VZ/ FZ - Pro Series

I've got something weird happening with the display of my VZ10m (its 
a Hohner HS-2/E actually but its exactly the same machine). Every now 
and then the screen seems to jump downwards and the bottom of the 
display jumps to the top as if the screen has rolled over somehow. 
Its only ever one line of characters that appears at the top. Another 
wierd thing is that occasionally the vertical lines of some 
characters seem to vanish i.e. the upright of a L or E character will 
disappear. 
Has anyone else ever encountered this problem and how do i go about 
fixing it? It looks like the symptoms of a dying internal battery or 
maybe a connector is loose inside but i'd rather see if anyone else 
has fixed this problem before poking around inside
Cheers
Paul