ouep, j'ai retenu ceci :
Graphical Mode on startup:
##########################
change the device section of /etc/X11/xorg.conf file to:
Section "Device"
Identifier "Videocard0"
Driver "fbdev"
Option "ShadowFB" "false"
# Option "UseFBDev" "true"
EndSection
EDIT : avais pas vu l'écran plasma, ...