Undefined video mode at boot.

Here's one choice little problem that remained after a failed partial upgrade to Ubuntu 9.04 beta.

After boot, the message "Starting up... Undefined video mode 31f" comes up, with a list of all supported video modes below it. You can press enter and choose the video mode which you would like. In my case, that's 37D.

Here's how to fix it:
Open /boot/grub/menu.lst in your favourite editor (gedit, nano) as su, i.e. sudo gedit...
Search for "vga=". In my case, it was vga=799. I replaced "799" with "893", rebooted and all was well.

In your case, you need to check what the hex number is for the mode you want. Google for a hex to decimal converter and convert that hex code to decimal. 31f = 799, 37d = 893, etc. Change the value at the first "vga=" instance to the value that matches the video mode you want, reboot and all should be well.

Cheers!!