I think it's quite possibly a problem with refresh rate. Many people are having troble viewing monitor screens under hi-res when the refresh rate is too low (eg. 60Hz or below). If it's refresh rate related it's mostly related to DirectX, your graphic card and your monitor. I don't know if you can override it within AGS, but you may try setting your desktop to 800x600 16bit once and set it to a higher refresh rate your monitor can handle (mostly selectable from the 'advanced" menu in windows' mode setting tab), you may set the desktop res back to your normal one afterwards. Now try to test these games again and see if that makes any differences.