Oversocialization?

I have a 9 month old Jack Russell who has recently been showing some pretty serious aggression towards unfamiliar dogs. I work as a trainer and he has been going to work with me everyday since he was 8 weeks old. Until just a few weeks ago he had such a stable and reliable temperament with other dogs that we have been regularly using him in behavior evaluations and to help socialize clients dogs.

However, he has changed a great deal in this regard in a matter of weeks. Most, but not all, new dogs he comes into contact with send him into an explosion of aggression with very little warning beforehand. He has become very unpredictable and somewhat of a liability. He has been fine with any dog that he already has a relationship with even if it's been months since he's had any kind of contact with them, and still LOVES to party and play with dogs he knows.

I have talked to a number of people, both professionals and non-professionals, and everyone seems to be a little befuddled by this behavior. At this point I see no signs pointing towards it being caused by a medical issue, he has never had any unpleasant experiences with unfamiliar dogs (to my knowledge), and obviously going to work with me everyday has been very well socialized with a great variety or breeds, ages, and temperaments. My concern is that being around so many dogs everyday he may have been over-socialized to the point where a constant influx of new dogs is stressful on him.

Has anyone ever seen or dealt with anything like this? Any advice or thoughts on the subject would be greatly appreciated!