If there are not major down sides please file a complaint with the FBI, I believe the url is:http://www.ic3.gov/default.aspx.
I encourage you to explain
* your evidence that when accessing various websites they appear to be tampered with between the server and your computer.
* Your worry that it impacts your bill with Comcast as it seems to be eating up you bandwidth. An estimate of the amount of money being eaten up if you have reason to suspect it is a city wide occurrence how much money is lost for everyone across the city?
* If you have packet logs of these occurrences I encourage you to include them.
* Unless you have hard evidence that points to Comcast that is doing the tampering I would not accuse any party of responsibility.
* If you have concerned friends who can independently verify similar conditions, it would probably be valuable to have them file similar complaints, referencing each other where applicable.