I am using Mountain Lion on my MBP, my external TFT monitor used to work great at its native resolution (1920x1080) when connected to the MBP.
However today after shutting it down and powering it up the resolution on the external monitor was on 800x600. In system preferences the 'Best for Display' resolution output was selected (wasn't that supposed to be 1920x1080?) when I clicked 'Scaled' and chose 1920x1080 the resolution indeed changed, however it is not taking up the whole screen. There are 1 inches to the left and right of the monitor that are not being used. Its as if the viewport is "shrinked".
I tried downloading SwitchResX but it did not help, when choosing 1920x1080 again the viewport is missing about an inch to the left and right.