Well, I have booted to the prompt before Xwindows starts and taken note of the refresh frequencies. I then started in failsafe mode again, but this time with the "knoppix vsync" and "hsync" operators set the same as they were at bootup screen, but no joy.

I'm no linux expert, but I'd say that Xwindows in Knoppix doesn't know how to drive a Radeon X1900XT's DVI port properly. Just a wild guess though.