Mandatory Restart - OS-X.2.8

[Ken Johnson]

My computer will periodically go into a freeze and demand a restart.  
When this happens, a text box shows up in the center of the screen, 
everything else, behind it, is dimmed.  Loosely quoted, the text, in 4 
or 5 languages, says "You must restart your computer now by holding the 
power button, or pressing the restart key"

It only happens (so far) when I'm on-line - but either e-mail (OSX 
Mail) or browser (OSX Safari).

Any thoughts on this one??

Ken