A longhaule flight in x-plane 11.50 and After ca, 5 Hours is navigraph dead.
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The stuttering/disconnect when Simlink exits can be caused by the Simlink plugin being a different revision to the latest Simlink. Best in Simlink Plugin Settings to uninstall Simlink plugin from each FS, then completely uninstall Simlink using Windows uninstall.
If they exist delete directories
c:\Users\username\AppData\Roaming\Navigraph\Simlink and c:\Program Files\Navigraph\Simlink
Then download and install latest Simlink using the default prompted locations.
If still an issue, please posts a screenshot of Simlink Status page using Posting Guide - Uploading Screenshots