TiVo HD doesn't receive HBO or Showtime when using cable cards

I have used my TiVo HD with cable cards for the last two years without incident. However, several weeks ago Showtime stopped working; all I see is a gray screen with cable card information. Additionally, HBO stopped working this week (and has the same issue). I have tried calling Comcast several times, but am on hold for far too long and then have been told that the problem can't be fixed remotely. However, I see that several people have had the same issue and reported it here, so I'm hoping that the issue can be fixed.

I also know many people who have experienced the same problem; almost every single person (including myself) is about ready to cancel their Comcast service. At the very least, service credits should be issued to everyone experiencing this problem. Major service interruptions are bound to happen, but I hope that Comcast will do the right thing and fix the problem.