Look in /etc/X11/xorg.conf, and locate a section that is labeled 'Section "Screen"'. In that section there should be a line labeled "Modes", that lists the various resolutions possible. If there is an entry "1024X768", then edit that line to make that mode the first one on the line. Save and restart X. If the entry is there, you should be able to manually switch modes with the 'Ctrl-Alt-Gray+' or 'Ctrl-Alt-Grey-' keystrokes, while the X server is running.