Co-op in this game is fine. Most likely cause is your connection itself. I have played for 3+ hours straight with someone and never had an issue. Only time I think I got a connection issue is in TIR where I think that is hosted on someone's machine or it quits out if someone else does.
Is there multiple people accessing the internet on either side? If so this lowers the bandwidth available that and could kill a connection, which is most likely the cause. Other thing is to test how the internet is on a computer during this "bad connection" timeframe to see whose end it is.
Phantasialove posted... Saving the game often disconnect your partner
Never had that issue but did see have it happen to others. I would guess bad internet or something to that degree as it doesn't make sense that saving a game to a local hard drive stops someone else's internet connection.