Can you post with what video driver does knoppix boot ? Did you try the xmodule=vesa ? When you use xmodule=radeon it's not guerantee that knoppix will use that driver - it will try to use it, but if it does not work it will fall back to fbdev or vesa. So if you boot a few times successfully using vesa or fbdev then it's really the video driver to blame. I still think there is nothing else to blame