First, be sure you have the latest version of the program, including the vehicles updates. If you don't know or are not sure, just download the full GSX installer now, that contains everything.
If the issue doesn't go away with the latest version installed, then it's nothing that has anything to do with GSX, and it's caused by a problem with your video settings. Try all these solutions:
- Update your video driver
- Reset your video driver settings to default
- Reset the FSX settings to default, by removing the following file:
%APPDATA%\Microsoft\FSX\FSX.CFG
This will force FSX to rebuild a new config with all default settings. After doing this, be sure you apply the HIGHMEMFIX tweak in the Addon Manager, save the settings and restart FSX.