X screen wraps around

[Duncan Sands <duncan.sands@math.u-psud.fr>]

I am running X 4.0.3 on an 8500 powermac (controlfb), kernel 2.4.
If I use 8bpp all is well (kernel parameter: video=controlfb:vmode:14,cmode:0).
If I use more bpp then the screen image is displaced to the right, and
wraps around onto the left of the screen.  Having the left of a window
on the right of the screen and the right of the window on the left of
the screen makes my head hurt...  However, having only 8bpp is a pain too.
Can anyone suggest how I can have more colours and less head pain?

Thanks,

Duncan.