I have a Mac which regularly runs into a problem. The user interface stops reponding, showing a "frozen" user interface. The mouse is still moving but clicking does not trigger anything. This happens about once a week. Solution so far is to force switch-off the Mac and reboot it.

I have ssh root access to the Mac. Killing (kill -9) the active application has no visible impact on what is shown on the screen.

Any ideas on how to diagnose this?

Is there a way to restart the window manager from the ssh shell? Killing /System/Library/Frameworks/ApplicationServices.framework/Frameworks/CoreGraphics.framework/Resources/WindowServer seems not to be possible.

The Mac is an early 2008 iMac and runs Lion with latest updates. /Library/Logs/DiagnosticReports is empty.

Update:

Problem stays after update to Mountain Lion.

The WindowServer process is in "uninterruptible wait" state ("U" flag in ps output set):

imac:~ root# ps ax|awk "NR==1|| /WindowServer/"|grep -v awk
  PID   TT  STAT      TIME COMMAND
   86   ??  Us    50:51.69 /System/Library/Frameworks/ApplicationServices.framework/Frameworks/CoreGraphics.framework/Resources/WindowServer -daemon

Any idea for diagnosing what blocks the process?

Any idea for "waking up" the process?