Hi,
I do have the same problem and the cause is the autofollow. I am not sure if it is a bug or a feature but I don't expect bliz to fix this. You can work around it by setting up two macros:

1.) follow mainguy. Hit the macro so every toon follows your main and IMMEDIATELY start moving your main.
2.) "unfollow" mainguy. Setup a key so all toons stop following the main guy: for me it is the move backward key.
As soon as you stop with your mainguy hit the unfollow!

The disconnects happen when the followers get into that "walking mode" when following. I have no clue how bliz messed up their source code for this to happen but my guess is that the 2.1 client keeps adjusting the position of the follower and that leads to huge data traffic.

Anyway,
follow only when the main guy is moving, unfollow as soon as he stops.
Hope this helps,
Bunny