Check you are not running FSX into any of the "Compatibility" mode. Right-click on the FSX.EXE program, select "Properties", select "Compatibility" and be sure nothing is checked there, which is how FSX is installed by default.
If there's no Compatibility mode enabled, then the problem is surely caused by a video driver problem or a setting or an FSX setting that shouldn't be applied.
- First try to reset your video driver settings to default. Be sure you don't use "Transparency Antialiasing"
- 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 have the "Advanced Animation" option enabled
- If that doesn't work, try to "Repair" your Acceleration or SP2 installation, this has fixed the problem for some users.