Pages 404 not found

Try resetting your custom permalinks back to the default setting and renaming your WordPress .htaccess file.

If that works, then try reading Using_Permalinks before setting a custom permalink structure again.

If clearing your Permalinks does not alleviate the issue, this could be attributed to anything from a web server configuration to a bad WordPress install.

I did permalinks resetting and its helps, but how to avoid this random problem in the future? It happens 1-2 times per month.

It there any way to troubleshoot it?
This issue bored already, now happens almost daily.

See if this helps: How to fix 404 error when using Pretty Permalinks?