This might help, but its probably too easy.
Have you checked your lan settings. Its worth a shot.
Device Manager --> Network Adapters --> (expand and select an adapter) -- (double click for properties) --> (select advanced tab). There are a couple of settings that will affect your shut down.
Enegery star: mine is enabled but you could temporarily disable and try
Wake on shutdown: mine is set to on, once again you can temp change it
Wakeup capablities: I have this set to magic packet because I have a network.
I fixed one that had an Asus board that was doing the same thing and tweaking those settings fixed it. You probably haven't changed any of them. But in this digital world things have a way of changing on their own. If you have dual lans disable the one you are not using. If you are using both the both will have to be tweaked.