after some investigations, it seems it is a well known bug.
Please find more information here : https://bugs.freedesktop.org/show_bug.cgi?id=2032
which concludes with
"the i830 and upwards ignores modelines and uses the BIOS to do the mode setting
so it can only use BIOS supplied mode lines...
you might give 855resolution a try so you can add the line to your BIOS..
google for 855resolution
it might also work with the latest Xorg CVS as well as Alanh mentions getting
more info from the BIOS....
we can do nothing about it as Intel don't give this info out...."
Would you have more information?
Thanks in advance,