Optiplex 960 hangs on shutdown

Hello.  I have seven Optiplex 960 SFF machines all running Windows 7 Pro, with HDs loaded from the same image.  Six of the units are fine.  One of the units will not power down all of the way using the shutdown command in Windows.  The computer will log out and then "Windows is shutting down" is displayed on the screen.  It will sit there with the same message on the screen for 10 minutes or so until it eventually reboots back to the login screen.  I have noticed that after about the first five seconds or so of the "shutting down windows" screen, the HD and Network LED on the front of the unit will go off for a few seconds and then the network LED will come back on...which leads me to believe that Windows is telling the hardware to power off, but it does not. 

I have updated to the latest A08 BIOS and installed the Microsoft KB977307 hotfix, but neither have fixed the problem.  I have also loaded defaults in the BIOS and run diagnostics (= no problems found) .

Have you fixed this problem before?  Anything else I should try?

Thanks!