If it doesn't happen on music, at least that's a good sign.
It could just be something on those particular shows you are speaking of. (bad recording or intentional ) What type of shows are you referring to exactly ? What programs ? Cable, movies, tv tuner ? Can you be a little more specific ?
Do you have or can you borrow another set of speakers to test this out and verify it's not your tweeters.
While there is always a chance it may be your AVR, I sort of doubt it, since you are having no issues with music.