Can't use high resolutions!!

[[EMAIL PROTECTED]]

Hi,
I've a matrox G400 32M and use VESA driver for it.
Well, the only resolutions I can see are those under or equal
800x600.... if I try to use higer resolutions the console crash
(graphically...). The problem doesn't seem to be influenced by
the number of color used.
What can I do?!?
Many many thanks,

                               Teo.