It sounds like when it boots it is not finding the correct diver for your video card and I bet it loads the vesa driver and it is not working properly. Post what kind of card you have and see if anyone knows how tell knoppix at boot to use the correct driver. I do not know how this is done but if I find something I will get back to you.

In the meantime try at boot typing: knoppix vga=normal
the resolution will suck but you can see if it works.