When you start up the computer holding the Shift key, the display stays in 640 by 480 resolution, and never switches to the resolution you set in the Monitors control panel.
Solution
When the computer is started with the Shift key down, all system extensions are disabled, including the display extension that loads the display preferences. To see the resolution that you set in the Monitors control panel, you should use the Extensions Manager control panel to enable the system extensions used by your monitor.