Reason for the issue:

1. Application Pool Idle Timeout: IIS uses an application pool to manage the lifecycle of hosted applications. By default, IIS automatically terminates worker processes for an application if there is no activity (idle) for a specified period (default is 20 minutes). This behavior is designed to conserve server resources.

2. Recycling Settings: IIS might be configured to periodically recycle application pools, which can cause your Node.js application to stop and restart.

3. Lack of Persistent Requests: If your application doesn't receive frequent requests, it may stay idle long enough for IIS to stop the worker process.

4. Application Initialization Settings: If the "Always Running" setting is not enabled, the application will not restart automatically when it stops, and users will experience delays as IIS initializes the application again when a new request comes in.

Steps to fix this issue:

1. Configure Application Pool Settings:

   1. Open IIS Manager.
   2. Go to the Application Pools section.
   3. Select the application pool your Node.js app uses and click on Advanced Settings.

2. Adjust the following settings:

   1. Idle Time-out (minutes): Set this to 0 to disable idle time-out.
   2. Regular Time Interval (minutes): If enabled, consider increasing the recycling interval or disabling it by unchecking Regular Time Interval under Recycling Events.
   3. Start Mode: Set to AlwaysRunning to ensure the application pool is always active.
   4. Ping Enabled: Set this to False to prevent IIS from terminating the worker process if it doesn’t respond to periodic pings.

3. Enable Preloading for the Application:

   1. In the site's Advanced Settings, set Preload Enabled to True. This ensures that the application starts automatically when the IIS server restarts or the application pool recycles.

4. Modify the IIS Configuration for the Site:

   1. Go to your site's settings in IIS Manager.
   2. Open Configuration Editor.
   3. Navigate to system.webServer/applicationInitialization.
   4. Set the doAppInitAfterRestart attribute to True.

5. Use Application Initialization Module :

   1. Ensure the Application Initialization module is installed in IIS. This module can keep your Node.js application warm and ready to handle requests, even after pool recycling.