I tried messing with both software and hardware firewall settings (I have a Netgear WGR614v10 router) and nothing seemed to help.
Finally, last night, I completely disconnected my router and ran my internet connection straight from my modem into my computer.
This has solved all of my problems.
I understand that not everyone is able to do this, depending on your specific setup. Nor should you really have to do this. But, I love the multiplayer in this game and it is worth it to me (for the time being) to go through the hassle of reconnecting the router whenever I need wireless network access.
Maybe I'm an exception and this won't work for anyone else. Just hoping I can possibly help someone who hadn't considered this possibility before.
Thanks