Shih-Tzu snapping and nipping

Question
My year old Shih_tzu seems to be very high-strung and unpredictable...he will suddenly snap his head around and nip at whoever might be petting/talking to him.   He has drawn blood twice on 2 different children.  When he was a puppy he nipped at ankles, hands, etc., but as soon as we got a female pup, he stopped that behavior.   He was neutered at 3 months.  He especially is likely to snap when someone is face to face with him.

Answer
Hi Mary Lou,  I need to know exactly what happens leading up to his nipping.  Is he playing, getting really rowdy, does he growl, raise his hair, anything like that?  What happens after the incident is over?  Let me know so i can figure out how to help in this situation.

Dawn