Try adding the line in red to your monitor section of your xorg.conf file:
Section "Monitor"
Identifier "Configured Monitor"
Modeline "1024x768_60.00" 64.11 1024 1080 1184 1344 768 769 772 795 -HSync +Vsync
EndSection
In this section you may also have to add your monitors horiz freq and verticle refresh rates. I am including part of my xorg.conf to show what I mean:
Section "Monitor"
Identifier "Configured Monitor"
VendorName "Unknown"
ModelName "Delta"
HorizSync 31.0 - 68.0
VertRefresh 56.0 - 85.0
Modeline "1024x768_60.00" 64.11 1024 1080 1184 1344 768 769 772 795 -HSync +Vsync
EndSection
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