Let's assume it's grub then. I'm not 100% sure but you could try this:
In the file /etc/grub.conf, find a line that starts with "kernel", and at the end of that line, add the setting: vga=773. Reboot. (If there already is a vga setting, this is the problem; change it to 773.)
http://www.mepis.org/node/2992
http://home.nyc.rr.com/computertaijutsu/grub.html