Not a problem of the scenery, most likely a wrong setting on your FSX.
Have you enabled any of the "Compatibility" modes for FSX ? If yes, turn them off, FSX shouldn't run with them, because it creates all sort of issues like that.
If it's not the compatibility mode, try to reset your video card to its default settings and if still doesn't change, try to remove your FSX.CFG file at %APPDATA%\Microsoft\FSX, and have FSX recreate a new one with all default settings.