I don't have the sedan, but I just got my new LX V6 Coupe. I had it out on the highway a little bit yesterday (still just starting to break it in so I've been avoiding the freeway for the most part), and I had it up to 80 - 85 a couple of times. It was as smooth at all driving speeds as if it was sitting parked in my driveway.

I have had the symptoms you are talking about in the past on other cars. Sometimes it was fixable by what you have already tried to have done. I have also been told that it can happen from an alignment issue. But I have also known a couple of older cars that there seemed to be no cure for short of a complete suspension overhaul. One thing is certain...it shouldn't be happening to a new Honda!