Then I flew from KLAX to KLAS (and just 30 miles to land, I get a pop up saying COUATL is failed and also mentioned Simconnect error and then I paused the sim to see COUATL is not running..
The Simconnect error was the only really useful info that was needed to understand the problem. Next time it happens, please provide the exact text of the Simconnect error message.
Check also if you have a Couatl.ERR file in the %APPDATA%\Virtuali folder, and post its content. If you don't, enable Logging in the "Troubleshooting" section of hte "GSX Settings" page, and the next time the error happens, check again that folder for the Couatl.ERR file and, if there's one, post its content.