Tienes que aportar algo más de información sobre qué gráfica es , vamos mas datos..

Pd : O probar con vesa de momento y cambiando la configuración que hayas tenido ejemplo :
# Xorg configuration created by system-config-display
Section "ServerLayout"
Identifier "single head configuration"
Screen 0 "Screen0" 0 0
InputDevice "Keyboard0" "CoreKeyboard"
EndSection
Section "InputDevice"
Identifier "Keyboard0"
Driver "kbd"
Option "XkbModel" "pc105"
Option "XkbLayout" "us"
EndSection
Section "Monitor"
### Comment all HorizSync and VertSync values to use DDC:
### Comment all HorizSync and VertSync values to use DDC:
### Comment all HorizSync and VertSync values to use DDC:
Identifier "Monitor0"
ModelName "Dell P780"
### Comment all HorizSync and VertSync values to use DDC:
HorizSync 30.0 - 85.0
VertRefresh 48.0 - 120.0
Option "dpms"
EndSection
Section "Device"
Identifier "Videocard0"
Driver "vesa"
EndSection
Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1024x768" "800x600" "640x480" "640x350"
EndSubSection
EndSection