Well desyncs happen generally because a players computer lags behind in processing all the AI code at the end of a turn, or processing all the graphics textures as the game progresses and there are more units on the map, ofcourse the larger the map the worse this will be.
Some people say playing with DX9 instead of DX10/11 helps on the graphics side, as well as lowering the graphics settings. Also I would not play on a map larger than needed for the number of players.
In internet MP we generally don't play with AI civs or City states, as this really adds to the end turn CPU loading, and in MP the challange really is playing people not the predictable AI.
We also tend to keep our games to 4-6 players as being a peer-to-peer game, each additional player doubles the data being sent each turn, and franking the game code being the train wreck that it is, can't handle processing alot of data each turn, even if you have a killer gaming rig, let alone if you have an average rig.
CS
