[X4U] Strange Screen Saver and Shutdown Behavior

[godfatherK]

During the last couple of weeks, my 10.3.8 system has been randomly 
kicking into screen saver mode.  I could be typing, moving the cursor, 
reading a webpage, etc.  All of a sudden, the screen saver will start 
without a reason.  I set the screen saver to start after 5, 10, or 15 
minutes of inactivity.  The only thing I can note so far is that 
shorter the inactive minutes set in the System Preference, this strange 
behavior increases and vice versa.

Worst yet is after the system has been running for (estimated) several 
hours, there is no way of shutting it down or restarting.  Choosing the 
options in the Apple Menu does nothing.  The usual control-eject 
keystroke doesn't even bring up the shut down / restart panel.
So far, the only suspicious thing that I have found is this entry in 
the system.log:
Apr  8 22:03:23 localhost diskarbitrationd[90]: loginwindow [262]:30211 
not responding.
And, the only way to safely shut down the computer is through the 
Terminal.

To be clearer, during the first few hours just after the start up of 
the system, shutting down / restarting the computer functions normally.
I've already tried repairing the permissions and made sure that the 
kiosk mode is not enabled.