A thought here. Sometimes the monitor is not being correctly detected, and the following has helped some to overcome this
Code:
gksu displayconfig-gtk
Scroll through the screen tab to find the manufacturer and then the monitor name, and, if the same are there, apply. This will rewrite the xorg.conf file much as has been set forth above to recognize the screen (monitor) and the correct resolution. Sometimes, the respondents on the various threads have not been able to find their monitor, whether by manufacturer or model, and have chosen a "generic" while inputting the desired resolution, and that has helped.