I was planing to exchange the server respectively distribut the services in the near future anyway, but I'm interessted in what causes the load now or where the system "hangs" :-)
That still does not get to the bottom of the issue, which of course would be nice.
You should be running some kind of performance metrics gathering. I would recommend "Munin" for long term monitoring. The only real issue with it is that it has a 5 minute window hard-coded so you cannot catch anything finer grain than that. But sounds like yours is much more than a short, spurious issue.
If you want finer grain, use sadc and ksar. The 2 below is collecting at 2 second intervals which is pretty tight.
/usr/lib/sa/sadc -d -I -F 2 /var/log/foo/bar
or on a 64 bit box
/usr/lib64/sa/sadc -d -I -F 2 /var/log/foo/bar
It will log system stats at 2 second intervals, until you control-C it.
Then you can view the file in kSar.
go to "File" then "New Window" from the new window go to "Data", "Local Command" enter the command sar -A -f /var/log/foo/bar That's it! Now you will have some pretty graphs to look at!