I can only confirm the problem is solved with one of the following fixes:
- First try to reset your video driver settings to default. Be sure you don't use "Transparency Antialiasing" (or "Adaptive Anti-aliasing" ).
- If that doesn't work, try to update your video driver too.
- If that doesn't work, try to reset your %APPDATA%\Microsoft\FSX\FSX.CFG file, to have FSX rebuild a new one with default settings
- Be sure you apply the HIGHMEMFIX tweak to the FSX.CFG, you can enabled it with the Addon Manager too, that's the only setting that really needs to be applied.
If it still not fixed, perhaps you applied some strange tweaks to your video driver that can't be reset by simply resetting the driver to default and/or reinstalling the driver. You might try a video driver cleaner utility, then a video driver reinstall after cleaning.
Or, it's possible the specific version of your video driver has some kind of uncommon bug, sometimes is not certain the very latest driver it's always the best one, depending on video card, some users have better results with FSX with older drivers. nVidia discussion forums are full of similar issues and suggestions about which driver release is better with a particular card with a specific game.