Myself and a buddy of mine have been going through all halo campaigns on legendary for the past few weeks. At random times, the game lags out and we are returned to the title menu. A specific example, we ran through the majority of gravemind for halo 2; nearing the end of the mission, we lagged out and were forced to the title screen, receiving zero credit for that mission, forcing us to start over again! Is there anyone else having this issue? If so what can we do to avoid this?
Unfortunately, this issue is both common and, frequently, unavoidable. Basically, Campaign functions on a P2P basis, so you must both stay connected to the Internet during the entire mission or you will lose connection to each other and lose all of your progress. Lag-outs such as the one you described usually occur when one or both players experience a hiccup in their Internet service that briefly causes a large loss of data. It makes the game believe that the players have lost connection to each other and ends the session.
The only real way to make it less likely is to improve the consistency of your personal Internet connection. You can try to use a wired connection, play at a less busy time (evenings after work are the busiest times, while late at night and early morning are not too terrible), or reduce the number of devices using your internet simultaneously while playing Campaign. Also, it is usually better to let the person with a stronger Internet connection be the host (i.e. choose and start the mission from the main menu), as it can improve the connection’s stability.
Sorry I can’t help more with this issue. I find it frustrating, too, as it happens a lot when I go through LASO with my buddies. It’s always stressful when the game starts getting really laggy. Hopefully you have better luck than me.