Do you know the make + model of your monitor, or have the manual lying around for it? It sounds like it can't correctly determine the horizontal + vertical refresh rates properly. You can fire up sax2 to reconfigure your monitor from the console. Simply press Ctrl-Alt-F1 after the system has booted and the graphics become distorted and log in as root. Then:
. This will go back through re-configuring your display, allowing you try auto-detecting your monitor settings again, or manually provide them.