Well, the database server is seperated from the webserver, and it does happen that I get the "Temp error because of no database contact" from Joomla.. Can't really say that it's very common during normal operation of the site though..
Sounds like the source of the problem.
During a database update when the database server takes too long it appears that the database transaction is not completed properly. That's why it doesn't happen all the time. I dont really know what can be done to handle this (from a software point of view).
Well, we've never had any problems with any other component because of this, like the forum etc. Also, the J! usertable is correct. A simple check of the returncode of the sql query is what I'd do in a normal computer software, dunno as much about php.