This issue is AT&T's fault. It is NOT a virus. It has NOTHING to do with peoples computers. It also does not affect all AT&T customers. People experiencing this problem are U-verse customers with the Motorola NVG510 modem. Here's what's happening and how to fix it -
THE PROBLEM:
When your modem loses its connection, AT&T has decided that instead of your browser showing you the generic errors about not being able to connect to the internet, it should intercept your browser request and redirect you to a help page with a friendlier message. This wouldn't be that bad if it weren't for a BUG in the modem firmware. When it redirects you to /cgi-bin/redirect.ha, *it doesn't tell your browser not to cache the page*. That's the problem. Even when your connection is restored your browser continues to redirect you because it has it cached. This is why clearing cache fixes it and why using HTTPS fixes it (browsers don't cache HTTPS).
THE SOLUTION:
You can disable the redirect entirely by enabling telnet on the modem and changing a configuration option.
This video shows how (step by step in the description) -
http://www.youtube.com/watch?v=o_O6rus8Yqw
All credit goes to Jordan Earls from earlz.net -
http://earlz.net/view/2012/06/07/0026/rooting-the-nvg510-from-the-webui