First, be sure you have the latest version of the program. You can check the file version for the fsdreamteam\Couatl\Couatl.exe file that, as of today, is supposed to be 3.0.3159. If your Couatl.exe is older than this, reinstall it using the current installer.
If you have that version, enable logging in the "Troubleshooting" page of the GSX->Settings menu. Restart the sim and, if the problem happens again, exit immediately, go into the %APPDATA%\Virtuali folder, and send the Couatl.LOG file you should have there. Don't post the file in a message, but .ZIP it and attach it to your reply. Or, send it by email to me.