I have 6 content sources in my SharePoint 2016 - all content sources have been running without any issues over the last 3 years. But suddenly we notice that out of 6, in 4 content source crawler is running fine (incremental and Full crawl) but 2 content sources are not running properly - the incremental crawls stuck for these two content sources - so, we have stopped this using the PowerShell, after this automatically full crawl starts for this 2 content source but it never ends, continue for more than 100 hours. Then if we stop again using the PowerShell script, it will start full crawling again but will never end.
Till now we have done the followings:
Restarted the search service and timer services in the application server many times.
Restarted the server couple of times
Cleared the cache from both the application and frond end server
(Following the Microsoft documentation)
Did IIS Reset
Did server restarts
After doing all the above, nothing helped us to fix the issue. Now, we request your help and suggestion to fix this issue.