Your monitor resolution & refrsh may be the problem. Try different setting by using the Knoppix Cheat Codes - http://www.knoppix.net/docs/index.php/CheatCodes
At bootup try: knoppix screen=800x600 xvrefresh=60
or knoppix screen=800x600 xvrefresh=60

Look here for the knoppix hdinstall howto: http://www.freenet.org.nz/misc/knoppix-install.html.