Desperate for help with random biting and snapping!

Hi!

I have really enjoyed reading everyone's posts, and I am hoping that someone can help me out ASAP. My husband and I have a wonderful (but boisterous) neutered lab/boxer mix. He is about one year and seven months old, and we adopted him exactly one year ago from the SPCA. When we first got him, though he was full of puppy energy and loved to chew things, jump up, etc., he was also a perfect dog for us, because he was very social and loved all people and all animals (dogs, cats . . . everything!). In the past year, he has been through basic obedience training, and we both work with him often. He gets plenty of exerciserunning and walkingand plenty of attention. At this point, he is a wonderful dog--he listens, obeys, and is almost always well behaved at home.

My major problem is this. About two months ago, our dog started becoming territorial (barking and growling at the neighbor across the fence and occasionally growling when someone new would come into our house), and in the last month he has growled and snapped at five people--even actually biting two of them! We take him everywhere (including the school where we teach):confused:, so this behavior is just not acceptable! I immediately bought "Sit, Stay, Fetch" hoping that it would give me answers, but I haven't really found the solution, because his aggression is so random.

First of all, the aggression (at least thus far) has only come out with men. Any woman could walk into our yard without a problem. Next, he will often be okay with a person, let the man pet him, and then suddenly growl and bite! The other night, we had friends over for dinner, and the dog growled for a moment when the man walked in. Then he was fine. About an hour later, the man was petting the dog and the dog was happy. The dog walked away for five seconds, turned around to come back for another pet, and suddenly growled and snapped at our guest! (Luckily, he didn't get him!) He did this same thing to the neighbor a few weeks ago.

To make the situation even more complicated, this behavior also happened twice when he was away from our yard on the leash. Both times we had stopped to talk with people on the walk, they had given him a pet, and then he had decided to snap at them. Most times he is petted with no problem at all! Finally, there is no logical pattern to who he decides to like or dislike. (Just the night before the above incident, we had a complete stranger over here with his three little dogs for several hours, and our dog was completely fine! And when he is out walking, he will almost always go right past people without giving them a second glance!)

Oh--and he still seems to have no problem with other dogs. He readily welcomes them into his yard to play, and he is usually not the top dog with them. He did get attacked by a pit bull who broke out of his yard about a month and a half ago. Our dog was on his leash and got pretty beat up, but he still seems to be fine with dogs!

We called a local trainer, hoping for some training help, and she said that the randomness of his aggression was really strange and worried her. She urged us to call the vetand we are planning to do that todayin case he has something physical causing the problem. Still, I don't get the feeling that this is the problem.

We are afraid now, to take our dog anywhere, because this behavior is so unpredictable. Most of the time, he is still great with people! Despite what the trainer says about taking him to the vet, like I said above, I'm not sure that is the problem, and my husband definitely doesn't think the vet can help. It seems that everyone but me thinks that we should give up and just take him back to the SPCA. It would break my heart to do this! Please, does anyone have any answers?

Thank you so much!
J1