Average ping is 100ms good? bad?
Well, from that you know that the physical machine - the server - is reachable and
running, but that's all....
It won't tell you if the GAME server (service, actually) is accessible/running correctly ON that machine, tho....
The truly curious thing about your problem is that you see it/get it listed in the Halo GameSpy Lobby, but cannot connect to it, but you HAD to connect to it to get the info from the game server?!? (You DO see the stuff scrolling by at the bottom about the Halo game config and players in-game, right?)