You may need extra polys in the nexck area. Also, if you used an old rig from Poser (I think Poser 2) then there's this extra bone in the neck that is invisible and not assigned to anything. It causes breakage (or did for me once). After that, I just started making my own rigs instead.  So yea, check the number of polys in the neck and chest  groups, then if that's not it, check the bone setup you used.