Me and my friend have been trying to play this game co-op. We haven't received CD keys from GOG, but I don't think they are necessary since we've been able to start and join each other's games. Now, he can start a LAN game and I can join it, as long as we use the fix provided in this post. However after about little bit of playing I will suddenly drop from the game. Has anyone else experienced this? Is there a solution?