Issue with HALO Multiplayer

[Greed]

Over the last week or so I've been having problems joining Halo Multiplayer games. The lobby loads properly, the map loads and the match is annouced, but rather than putting me into the game the monitor fades to black and the network error signal at the bottom right hand starts flashing. I've done everything I can think of; reinstalling the game, disabling other programs to preventing them from blocking my connection, etc. But what is driving me nuts is the fact that everything is working properly! I can still play other multiplayer on other games, the single player campaign still runs perfectly, and I can still host matches (as far as I can tell, the map loads but I haven't gotten other players on). Also as far as I can tell my internet is running as fast as ever. I've heard about other players getting stuck with this problem but I was wondering if anyone might know whats causing it. Any help would be greatly appreciated. Thanks.