Quote - I'm not sure it's the same thing you're describing in the above quote, but it looks like the edge of the head, where it welds to the neck, may have been moved in places.  You can get rendering or display artifacts when the child (head) weld vertices have been pulled away from the parent (neck).

What Cage says sounds like a likely cause. Just one small point though, the problem seems to be happening at the boundary between the neck and chest, not the head and neck. Did you morph the neck?