"Stop using your ISP's DNS" is not always the right answer.
If I use Google's public DNS servers it breaks content distribution networks like Akamai. For example, if I use Google Public DNS then *.phobos.apple.com addresses for iOS and OSX updates resolve to IPs inside NTT (Tokyo, Japan) and downloads are exceedingly slow (high latency, dropped packets, etc.). If I use my ISP's DNS server it just so happens they host a set of Akamai nodes and the *.phobos.apple.com addresses will resolve to IPs only 2 or 3 hops away, so downloads will saturate my SDSL connection.
Comcast should just acknowledge that they've fucked up and fix their servers.