Thanks for your quick reply!
I didn't know that Knoppix included memetest86...

So I let it run for the last 15 hours, then i cancelled it because it didn't find any errors whatsoever and I had to work.
It seems to be my graphic card!

Then again back to my question... Since graphics in Knoppix look fine when I boot in failsafe mode - only all the other hardware isn't supported - is there any way to boot knoppix without graphics but with support for everything else (and still work in x window)? Or is there any way to get the needed hardware recognized (usb etc...) in the console after booting in failsafe mode?

Thanks again,

Leif