My dog does this too! I've caught her with this head tremor about 4 times in the year that I've had her, however I work most of the day so there's no telling how often it really occurs. When I adopted her, she was in pretty bad shape, both emotionally and physically. I'm wondering if it could be psychological? I just took her for a check up recently and they said she was super healthy, but I do not remember having any of these tests done. Today I was actually with a friend when it happened so I know it is not just in my head! I'm not sure her breed, but she looks a lot like something called a carolina dog that I found on the internet. Any advice would be helpful!!