Fix Ubuntu/Plesk stuck minutes after server starts -- 2

I have a server that works fine for a few minutes, maybe hours, and then stucks, some services still up, some others not, i.e. websites are ok and mail not.

Using Plesk tools there are some errors and some are fixed other can't be fixed.

I need a sys admin for Linux that really know how to handle Linux or Plesk problems, so the server can work fine again.

Thanks.