Walks/obedience?

Question
Hi, I have a 2.5 year old black lab.  He's a very sweet dog, which we adopted when he was 9 months old.  He's had no professional training, but we've trained him in basic commands in the house.  We walk him on a prong collar (he hates the harness and the gentle lead) because he has gotten out of his collar and is much stronger than me.  Not often, but on more than one occasion, he seems to get possessed (and this only happens outside), where he grabs the leash in his mouth and pulls really hard, trying to wrestle it away from me, so he can take off (he's a runner).  it seems like very aggressive behavior, where he knows he's able to wrestle it away from me, and he's only done this with me, not my husband.  Today, for the first time, he did it even with the prong collar - he took the leash in his mouth, and pulled that way.  I don't know what triggers this or how to stop it, but help/advice on either would be greatly appreciated.

Thanks in advance!

Answer
Neutering him if not already may help.  Unless he is part of a serious breeding program, there is not good reason not to neuter him.  

Did you have help fitting the collar?  If not fit properly, they can be dangerous or ineffective.  

You may need to do some reading.  You want to stick to positive methods, but not get bogged down in psycho babble.  Youwant the dog to respect your leadership.  Try Monks of New Skete and their The Art of Raising a Puppy, ISBN 0-316-57839-8 or How to Raise a Dog When Nobody is Home, Jerry Kilmer, Puppy Primer, by Brenda K Skidmore and Patricia B. McConnell, Ph.D.,Beginning Family Dog Training, by Patricia B. McConnell, Ph.D.