[mdlug] computer shutting down

[Robert Meier]

Wolfger,

> You likely have one of two problems, ...

  o power-supply
  o motherboard overheating

I agree these are most likely.

> Can anybody tell me how to diagnose this problem?

<SUGGESTION>
After reboot:
      tail /var/log/messages
look for the events occuring within the hour and minute before shutdown.
It is probable, that whatever fails, recover is attempted, and though
unsuccessful, a message may point to the culprit.

Increase your logging level set in /etc/syslog.conf.
See syslog.conf(5) for further details.
(Depending on you distro, you may need only to uncomment the lines
containing /var/log/warn.)

Though improbable, check your wall power.  Plug the computer into
a different circuit (i.e. controlled by a different breaker).
Plug a digital clock without backup battery into the current computer
socket.  If it blinks 12:00 in the morning, get an electrician.

Though improbable, when did you last change your computer cmos battery?
In most of the Dells I've serviced, I try to change the battery
annually.  When circumstances prevented changing the batteries, the
users reported "flakey" annoyances starting after about 2 years.
</SUGGESTION>

Hopefully helpful,
-- 
Dr. Robert J. Meier