If it does turn out to be a DNS server issue with Comcast, you do not have to use Comcast servers for DNS, or Google's or Level3's.
I recommend you run some tool such as GRC's DNSBench at http://www.grc.com/dns/benchmark.htm or Google's namebench at http://code.google.com/p/namebench/ to find the fastest DNS servers for your location. I would also rerun these tools periodically because things do change.
Lee