Happens a lot with laptops and some exotic vga chipsets...

Try the one of the screen optionsfrom the cheatcodelist .
( http://www.knoppix.net/docs/index.php/CheatCodes )

In your case try :

knoppix xmodule=fbdev knoppix xserver=XFree86 fb1024x768

Hope this'll get you on track .

greetzzzz,