Something else I just discovered is you can check Xorg (the subsystem that runs keyboards, mice and displays) for messages. For example I changed my built in display from 1280x800 to 1024x768 like yours. Then I changed it back to 1280x800 and looked at the subsystem's log file:
On your system change the "--lines"=1 to "--lines=20" or something large enough to see the error messages, if any, that Xorg reports. Note you do not type the "~$" which is the command prompt on my system.
~$ tail --lines=1 /var/log/Xorg.0.log
[ 5339.418] (II) intel(0): switch to mode firstname.lastname@example.org on LVDS1 using pipe 0, position (0, 0), rotation normal, reflection none