I have exactly the same problem with Knoppix 6.1.
My machine:
CPU: Athlon 64 64 x 2 5000+
Video chip: nVidia GEforce 7050PV / nVidia nForce 630a integrated on MSI motherboard
Monitor: Benq FP73G, refresh rate: 56 - 76 Hz, native resolution 1280 x 1024
Dual boot: OpenSuSE 11.0 plus Ubuntu 8.10 Intrepid.
Both use a proprietary nVidia driver running at 1280 x 1024 resolution
with a 60.02 Hz refresh rate
PuppyLinux also runs perfectly under the same conditions.

When I boot Knoppix 6.1, however, things work fine till the system goes to level 5, then the monitor
displays an "out of range" warning. I have tried booting with "knoppix screen=" at various resolutions,
as well as defining various horizontal and vertical refresh rates, nothing makes any
difference.

I can open a new terminal in text mode (ctrl + alt + F1). if I enter here "xrandr -q" I get
the message "can't open screen". The X-server doesn't seem to be running at all.

I have read Klaus's answer to a somewhat similar problem in Linux Magazine issue #103.
This is of no help because the system doesn't even start at low resolution.

Actually I'm not all that worried because Ubuntu and OpenSuSE are running smoothly.
However, it seems a shame to give up on Knoppix altogether.