This is usually the result of faulty or overloaded DNS servers.
DNS servers provide the ability to translate alphanumeric internet addresses (ie. www.broadbandreports.com) into numeric addresses (ie. 188.8.131.52) so that they can be located across the Internet.
By default, Comcast automatically assigns you three DNS servers when your cable modem first connects to the Comcast network on power-up. Many users find their Comcast assigned DNS servers behave erratically and chose to manually configure their own.
If you would like to manually configure DNS servers, please refer to this FAQ topic for instructions on how to change your DNS server settings.
*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.