Peux tu essayer ce xorg.conf ?

Section "InputDevice"
Identifier "Generic Keyboard"
Driver "kbd"
Option "XkbRules" "xorg"
Option "XkbModel" "pc105"
Option "XkbLayout" "fr"
Option "XkbVariant" "latin9"
EndSection

Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
EndSection

Section "Device"
Identifier "Configured Video Device"
Driver "vesa"
#Driver "ati"
#Option "IgnoreEDID" "true"
EndSection

Section "Monitor"
Identifier "Monitor0"
Modeline "1680x1050_60.00" 146.2 1680 1960 2136 2240 1050 1053 1059 1089
# Option "PreferredMode" "1680x1050_60.00"
EndSection

Section "Screen"
Identifier "Screen0"
Monitor "Monitor0"
DefaultDepth 16
SubSection "Display"
Depth 16
Modes "1680x1050"
EndSubSection
SubSection "Display"
Depth 24
Modes "1680x1050"
EndSubSection
EndSection