I have the same problem with w7/ie9. It happens after the router malfunctions, decides it needs to reboot and you get that your internet connection is having problems diagnostic page. The page that you were on then is stuck always going to the "diagnostic" cgi-bin redirect page. To fix the problem, I go to "InPrivate" browsing and open the effected webiste where there  i can view the page normally. After that I can open IE back up in normal mode and the pages opens.

Clearing the temporary internet files and flushing the DNS entries do not fix the problem. I haven't tried to see if deleting cookies does however that would be a poor fix if it did.