You don't have any hand or arm symptoms with this?
Any previous whiplash type injuries, desk job, any previous RSI's?
How is your upper body posture?
head/shoulders forward of the body? rounded or hunched?

These factors may point to TOS - thoracic outlet syndrome.

I suppose the xrays/MRI would have shown if a extra cervical rib was present..but sometimes they are missed..
For more TOS info here is our forum for it-
http://neurotalk.psychcentral.com/forum24.html

Some of the therapies we post about may be helpful even if you don't have TOS.

Check out the trigger point sticky thread, if no one has mentioned trigger points to you during your appts/therapies.