Hey guys,
I know this is an old thread but I'm having exactly the same issue, 1 line display that shows first 8 chars on line 1 and then next 8 at postion 40... not much fun!
Did anyone ever work this out it must be an init setting, my LCD is part of a kit from maplin and did work as a 16 char display in that project (do not have the code for this 
) Model is GDM1601C
Thanks for any heko you can give,
G