The tv handles all of the other connections fine, incuding BluRay. It's only 4 years old. I think that the culprit is the cable box (it's an Xfinity Cisco RNG200N), which, after doing some further research, looks to have other people reporting this problem.
Unfortunately, I spent money upgrading my receiver that I probably didn't need to spend, but I go a pretty good deal on it, so I can only be so unhappy. I'll call Comcast and see how they handle it and hopefully this will go away.
Thanks for your help.