You might try one of the cheatcodes at boot.
e.g. "knoppix xvrefresh=60" see this: http://download.linuxtag.org/knoppix...cheatcodes.txt
for more cheatcodes. You could experiment with those codes that relate to your x-display to see if
you can get a usable screen and then configure xfree86.

I don't know what version of knoppix you're trying to boot but I think these should work.