Arrrrgh, this is killing me. I read about a problem like this on ReefCentral.com about 2 years ago. The problem was also happening with Candy Cane/Trumpet/Caulestra (whatever you want to call it) corals as well. Do you have any of those in your tank?
The problem would start with a portion of the colony not expanding as much...and eventually the heads would lose tissue...then the tops would float away. The rest of the colony sometimes remained unaffected or would have the same problem. Some of these unattached heads would land somewhere else and regrow...but most died.
I have ABSOLUTELY no idea where that thread is anymore. I don't remember the conclusion of it. Some people thought it was a lack of a certain trace element...and other people thought it was an "asexual" reproduction-type spawning.
I don't know what to recommend to you. I don't want to say "wait it out and see if the above happens" b/c by then it might be too late. I would check all over the colony for signs of any sort of pest...any sort of fish that might be annoying it (any angel fish?)...or maybe a small aiptasia pestering it. I would check at night to see if anything comes out. Try a few water changes (not one big one.) Try running some carbon as well to see if it's chemical warfare.
Sorry we can't be of more help. Can you post a picture maybe?