This has been a common occurrence for many friends and players I know and played with. The lag during the co-op campaign mission are ridiculous to the point where it’s not even worth playing. You can barely move! Just recently (About 5min ago) some friends and I were going for the achievement, “Vidmaster Challenge: Annual”. All of us knowing that it was going to lag, we coped with it and made it to the end of the mission where we made the jump. As we became closer to finishing the mission, the lag continued to get worse and worse. Eventually to the point where it said someone had left the game. Nobody had been disconnected though. We’ve tried different hosts, but nothing has came to a solution. It’s the game. Fix Halo 3 Co-Op Campaign Bungie!