I have two border collies with OCD and an Australian Cattle Dog. I am not a professional trainer, but I have done much dog training and had private sessions with professionals do focus on specific behaviors.
1) Find another trainer. Not all are the same. I really have trouble believing that an experienced trainer has never seen this behavior before. Think about how fruity celebrities can be? I did a search here (
http://www.apdt.com/po/ts/us.asp) and found numerous in your area.
2) Exercise, exercise, exercise! Not just walks. I mean run him! Fetch works wonders. One border collie of ours can be mad with energy (mad as in crazy). Every evening we play fetch until she plops down from exhaustion. It is also a mental outlet for dogs. Simple walks wont do it, however, one long ass walk is infinitely better than 2 or 3 short ones.
3) Obedience. Sit, Down, Stay, off, and watch me (get the dog to look at you in the eyes no matter what they are doing) are very important. More mental stimulation for the dog, plus it reinforces your position of top dog.
4) Training. Teach him that it is OK and good things happen when you leave. The trainer will help you with this. Whatever advice I give you here might contradict what the trainer tells you. The book I'll be home soon might help.
http://www.dogsbestfriendtraining.com/books-retail.php
Also, if it doesnt happen but once in a while, or sporadically, you might want to consider that it may be some specific, intermittent, stimuli that is causing it. Fear, for example. Thunderstorm, big trucks, motorcycles, cries from children playing.
Keep talking to your vet about it. Dont give up on OZ.
Our border collie, willow, was a problem child. Scared of strangers (especially men) and didnt trust me (got her at 2 months). Now she will let a stranger at the dog park scratch her neck and my wife calls her my girlfriend. Shell wiggle and jump into my arms. That being said, strangers in the house are very different. There is no such thing as a perfect dog cept in Disney movies.
BTW I didnt get a chance to read the whole thread (at work and I could only skim) so if Im totally lost with this post, forgive me.