I am see some random complains with Level 3 DNS. I don't know if this is a temporary problem or chronic to being overloaded. As a Level 3 customer we have an open support ticket but have not gotten a satisfactory answer from Level 3 engineers. I am recommending changing your routers, PCs to use Neustar's DNS at:
156.154.70.1
156.154.71.1
http://www.neustar.biz/services/dns-services/free-recursive-dns
An alternative would be use Dyn DNS which has become very popular over the past year
216.146.36.36
I would NEVER recommend using AT&T or TW Cable's DNS as they are very slow, time-out and have very long TTL times. You will find other Free public DNS servers and reviews at http://pcsupport.about.com/od/tipstricks/a/free-public-dns-servers.htm