Not a problem of the scenery but, most likely, video memory corruption that happens only after a flight, likely caused by a bugged video drivers, an outdated video driver, a tweak in the video drivers which shouldn't have been applied and a tweak in the FSX.CFG that shouldn't have been applied too or it's missing. Or, even a combination of all these issues.
- First try to reset your video driver settings to default. Be sure you don't use "Transparency Antialiasing" (or "Adaptive Anti-aliasing" ), as explained on the scenery manual.
- 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.
- Be sure you apply the HIGHMEMFIX tweak to the FSX.CFG, you can enabled it with the Addon Manager too.