Logs please covering time when it becomes unavailable
Ensure debug logging enabled
See https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/
restart the server to get fresh logs set created
when the issue arises, note down the times and capture logs
See https://support.plex.tv/articles/200250417-plex-media-server-log-files/
Attach logs together with details of the problem including times
