Have a look at the Cheat Code documentation here: http://www.knoppix.net/wiki/Cheat_Codes

I would look at the xvrefresh and xhrefresh options and try to specify parameters that are compatable with your Monitor. 60 hz might be a good starting point. There is a cheat code that worked in the past for some monitor issues, "knoppix noddc" that may not be in the documentation. It's also worth a try. Be careful to use the proper syntax when using cheatcodes. You usually need to type "knoppix" and then the cheatcode(s). There are a few examples in the Documentation at the above link. There are more subjects covered in the "Documentation" link at the top of the page. It makes for some interesting reading.