I remeber DAOC used to have similar issue. Before it has to do with the background not getting enough resources and ends up DCing.

When I started multiboxing wow I was disconnecting too. Mind you I have a quad core with 4 gigs of ram and such. What I do to fix my problem is I close as many background process as possible. I load in windows mode. Then I set the pority for my wows one setup higher. I make sure the none forground wows are minimized. That seems to work for me. I can have 3 wows running with no dc and /follow. Anything more and I'll get random dc in mins.