George_Fitzgerald,
ISPs occasionally have problems with the configuration of their DNS servers, which are the servers that work "behind the scenes" to convert the "name" of the website you want to visit to its IP address.
Please try an alternate non-Comcast DNS server to see if that helps matters. I can suggest trying the settings at www.opendns.com Most home and small-biz wireless routers allow you to specify the DNS servers.