Black lab aggression

Question
Let me start with some background info about our family dog. When he was a little over 2 he was stolen from us and returned 3 months later (for a reward). During the time he was gone we bought a yellow lab puppy.Upon his return he began marking inside our home and once marked our older dog as he lay sleeping on the floor. We had him neutered shortly after because the vet recommended it and said it may help. A year ago we moved in to a new home and this is were our problem began. Our black lab is now 5 and our yellow lab is 4 and not neutered. Shortly after moving in to our new home on property our black lab has been challenging our yellow lab (who is not aggressive)and fighting him. Our yellow lab wins every fight. The fights used to only happen after feedings but now happen for no reason and are more frequent. The fighting has even occurred in the house. We have contacted trainers and lab rescues for answers. We've tried several different things but it seems to resort back to fighting. We have two small children who I fear will be near when a fight breaks out. What do we do? Our black lab is not aggressive to people just our yellow lab.

Answer
That is really outside my experience.  It is possible the yellow Lab is challenging him in subtle ways you don't see, body position, eye contact, head position, etc.  The black one then fighjts back.  If so neutering the yellow might help.  

I am sorry I don't have a better answer.  Perhaps a better trainer might help.  See www.apdt.com to help find one.