This started to happen after i made a system restoration and got myself 2 new mods. The mods where removed but it still happens and the only thing i found on google was someone asking if FWE was the reason for this but no one answered the guy.
This happens to any NPC and it's random when it happens. But once every 7 or so NPC's seems to be doing so if they are running or in combat.
Remember kids, sentences are like girlfriends. If the period doesn't show up, you're in trouble!