Are you all shutting down all non-essential programs before starting the game, especially anti-virus and anything network related?
If you create a new profile and start a new game, do you also get frequent disconnects? Or have you tried loading an old save from when it was working?
Have you tried switching who was hosting?
Have you tried playing in pairs, or with other people, to see if you can narrow down where the problem is? The issue could be with a particular system, a certain combination, or with the number of players, etc.
