Just writing to finish this thread.
I've been reducing the ranges of Hrefresh and Vsync without any luck, those lines keep appearing. But I don't mind, they aren't annoying at all.
One thing though, this "problem" is gone if I set it to 1024x768 but my card can display it only in 8 bit, which looks horrible...
Finally my videocard dislays with the vesa driver (IIRC
|8 bit|| X|| X|| X|
|16 bit|| X|| X|
|24 bit|| X|
Thanks for all the help.
PS: The default login screen looks bad in lower resolutions (640x480,800x600). Eg: I am unable to see the "login" and "password" words. It's like they got behind the, I don't know how to call it, the box where I enter the login and password.