Be sure you don't run FSX in "Compatibility" mode, because this play havoc with video drivers settings, and be sure you don't have "Transparency Antialiasing" enabled, because it might be bugged with some drivers.
If that doesn't fix it, try to reset your video drivers to default values, and if that doesn't fix it too, reset your FSX.CFG to its default settings, by removing (or moving to another folder) the %APPDATA%\Microsoft\FSX.CFG file.