weird lab behavior

Question
we adopted a black lab mix form our local humane society in the may of 2005  was raised with a young female boxer  first problem was food aggression had to feed the pup by itself or would try to bite the boxer  seemed to over the food problem but now is growling at us  we have raised him in a loving environment  is so friendly but has times when we are petting him he looks at us as he doesn't know who we are starts growling has shown his teeth then gets scares and hides under something  wen love this dog but cant keep a dog that we are afraid might hurt someone or his own family wondering if something might be going on inside is head that he has no control over will be taking him back to the shelter for evaluation please is this something that you think might be corrected or if this could be a medical problem in the brain that are common in labs we love this dog and only are concerned for his best interest

Answer
His problems may be a combination of genetics, early socialization, and a lack of leadership on your part.  You have discussed the problem with the vet?  Fortunately you maybe able to fix the problem.  The evaluation at the shelter sounds like a good start.  It is great if they have good people doing follow up work with their placements.  some dogs may fit into their new home fine, While other need a little help.  Often before a dog is 3 years old and mature, they have problems challenging their owners for pack status.

The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm  In a more difficult case like this, it would be better to attend a class where a good instructor can give you more help that a book or anybody on the net.  Yes you have a problem.  Can it be solved?  It is worth trying.