If all are going out at the same time its not bandwidth its cause to much data is going to a single ip and Blizzard software is auto disconnecting him. If I try and log in with all at the same time that sometimes occurs to me.

I would think with even the most modest internet connection 40 wow's can't saturate your bandwidth, and if that were the case not all would disconnect at the same time, also I think he would go AFK for awhile before the disconnect. I have to hit a "move forward" key on my guys when they go afk (and they do all the time even during battles cause they are always auto follow).

Maybe you can just put a short time (like 5 minutes) on your router to change dynamic IP address ... not sure if thats a good idea though.