well, you have to provide correct refresh rates for your monitor.
your xorg is not correctly configured. best you can do is to try to
connect in a root console. I don't know about how to do that
in ubuntu, but if someone can tell you here, do that, than when
you are root (superuser) type 'xorgconfig', hit the enter key
and answer the questions you'll see on screen.
have your monitor specs ready and when prompted, enter them.
hopefully at reboot you will avoid this annoying message.