I experienced a somewhat similar issue yesterday on my network that I described in detail here [0].
Essentially one of the computers (running ubuntu) on my network started sending a VERY high volume (it measured 20gb for the day, and I think it was all over a 10 minute period) of DNS traffic to my router, which runs an unbound instance for my network. That traffic (or at least I think it was that traffic) brought down my network to the point where I could even ping an external or internal ip address.
Does tcpdump show the destination ip address the traffic was sent to on AT&T's network? Curious if that could be a dns server..
Also, what version of ubuntu is your desktop running, and what software does it have on it? Are you using canonical's livepatch service?