Cannot stay connected to Horizon Life, most of the time can't connect at all.

90% of the time I can’t connect to Horizon Life whatsoever. If I keep hitting “Retry” on the error message, eventually I’ll find a Horizon Life session, and I’ll be able to see other drivers and go Head-To-Head with them and stuff, but only for about five minutes before I lose connection/get disconnected.
I’ve already identified that Teredo is “unable to qualify”, and I’ve done everything that the official support forum suggests except for factory-resetting my router; I share the wifi with several others and I don’t want to mess with it. But, the fact that I’m able to connect for a few minutes at a time suggests that Teredo is able to work, right? I’m just at a complete impasse as to what the heck I need to do in order to play online. I’ve never had a problem connecting to online modes in other games, but of course Microsoft wants to over-complicate the way their special game works and make simple stuff difficult…

Almost a week later and no replies? Some “community support”, huh.