On the following sites you will find data concerning the diagnostics of cervical spine instabilities that I personally, or people I know have experienced.
Overall, I feel a combination of Upright MRI and CT scan in rotation showed the most useful results.
Some doctors (as well as few literature) recommend the usage of a muscle relaxant before imaging. This should relax the protective muscle spams and help to show the full degree of instability, which could be hidden by muscular compensation.
My personal experience with different test you can read here.
Experiences with tests written by others are here.