The infamous Skele DC.

I wasn't affected, but I was in the skeles mini-dungeon the other day for about 20 minutes with a bishop and marksman from my guild. (First time I've been in there, and likely the last for a while, ha ha... 117 ain't exactly the time to be training on skeles.) The MM disconnected after maybe 10 minutes in there, while the rest of us were fine. He was not visible on my screen when it happened, so I have no idea what he was doing, though I'm sure he had his summon out. Later I heard him complaining in guild chat about more disconnections there; it seems to happen to him a lot, though at random times.

Edit: We're both on Vista, and I don't know about the bishop.