Don't know if this will help, but I just went through the same issue and
found that my problem was due to a 56K phone modem that I had recently
installed (in addition to the on board installed NIC for my external cable
modem). The phone modem was working fine and showing no conflict but my
sytem would only restart and never shut down until I removed the phone
modem. Once the phone modem was removed, the shut down went back to correct
operation. Maybe check to see if some hardware you may have recently added
might be causing this. Good luck.
