I have a netgear router/firewall, and there was a time for a few weeks where it was preventing me from connecting to IRC. I could bypass it and get on fine. I rebooted it, flashed firmware, did a bunch of crap, and it still wouldn't work, was giving some error about not enough memory to support the mapping of 6667 or something similar. Then after about 3 weeks it just started working again randomly.
Can you access any error logs in the comcast router? I assume it is also your modem which makes it difficult to bypass, but maybe you could open a VPN tunnel and test it over that.
http://sonic.net/hotspots/portal/ (if you still had sonic.net
I might be willing/able to get you a VPN connection short term for testing. Let me know.