If you are using a Dell P780 monitor, then according to Dell's tech specs
your vertical and horizontal refresh rates do not reflect their ranges. Your xorg.cong shows that they are set to:
Horiz = 31.5 - 50
Vert = 40 - 90
but Dell's info shows that the ranges are:
Horiz = 30 - 85
Vert = 48 - 120
You might consider editing your xorg.conf file to reflect those values. DISCLAIMER: Using video settings that are incompatible with a monitor's capabilities may damage it. As the LQ Rules state, do NOT make any changes unless *you* understand and accept their effect and possible impact.
Note that according to the documentation, this monitor's "optimal resolution" is 1024x768@85Hz, so trying to use 1600x1200 is therefore *not* optimal according to the manufacturer. Personally, based on that comment from the manufacturer, I would leave things alone. If you want to do otherwise, it's 100% up to you, so proceed with caution. -- J.W.