Not a GSX problem but most likely an FSX tweak or setting that is causing issues. As discussed other times in the forum, try all the following:
- reset your FSX.CFG file to default, by removing it or moving to a different folder ( located at %APPDATA%\Microsoft\FSX )
- reset settings in the video card control panel to default
- be sure you don't use the "Adaptive Antialiasing" option, also called Transparency Antialiasing, because it causes issues partial scenery objects missing.