The VGA bios sometimes does not work, and my suggestion is trying to cat the VGA bios under Linux shell.
What do you mean exactly? If you mean running the command advised at the end of www.coreboot.org/VGA_support, I've already done that with the bios shipped with my mainboard and the last version of it.
should I look if the images changes (with md5sum) between 2 extractions after reboots ?
Denis.