I wonder if it is only position related?? might be spinal c- spine maybe?
I don't remember if you have gotten any MRIs of C & T spine already or not.
or do you just notice it more when bending over?

possibly could be a trigger point and when bending over tightens the muscles that has the trp in it and that triggers the referred pain..

http://www.triggerpointbook.com/headache.htm
http://www.triggerpoints.net/trigger...idomastoid.htm
http://www.pressurepointer.com/trape...ger_points.htm
http://www.pressurepointer.com/Poste...ger_points.htm
http://www.pressurepointer.com/SCM_trigger_points.htm