It sounds like knoppix is doing the best it can at autodetecting the video hardware (perhaps getting it wrong) & then setting a resolution & refresh rate that is beyond the capabilities of the hardware. Try booting knoppix with some Cheat Codes, such as:
knoppix screen=800x600 xvrefresh=60
knoppix screen=640x480 xvrefresh=60

I hope this helps.
James