Hi I previously had a problem on my wife's computer with a Nvidia card,
in that X chose a mode that made the screen ghost from a live debian
boot. An incompatible mode was chosen.
I fixed it as follows:
I added the boot parameters : nomodeset vga=normal and it now works.
I cannot find this boot parameter in any documentation, but found it in
a pre-build image.
Hope this helps anyone with the same problem.
Rob Key