Try these settings in the Monitor section of your xorg.conf:
Modeline "1600x1200" 160.96 1600 1704 1880 2160 1200 1201 1204 1242 -HSync -Vsync
Those values above for Modeline should all be on the same line, that is, Modeline should include everything up to & including that -Vsync
Hope this helps.
You can thank Kingsnake for his post at linuxquestions.org