Francois, Try putting these two options into your XF86Config-4 into your Device section:
Option "XaaNoScanlineImageWriteRect"
Option "XaaNoScanlineCPUToScreenColorExpandFill"
It sure looks like hack, and I don't remember how I found it, but
.1 XFree release ago it helped me on Dell Latitude CSx with Neomagic NM2360.
Jacek
