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Is this the left forehead?
Back of the head?
Any neck pain?
How long does it last? Seconds?
Did you have any skin changes there recently? Rash? Infection? Swelling?
Does it hurt if you press down in that area?
Is it the same exact spot over and over?
Do you happen to sleep on that side of the head?
Well, the fact that this is a sharp, intermittent pain, in the same location, which comes and goes -- makes it seem that a nerve in the scalp is being irritated.
Such specific nerve irritations would cause a sharp, well defined pain (in contrast to more headache syndromes, which would be a more diffuse, longer lasting dull ache in most cases, or the sharp eye icepick as in cluster).
I would think that the nerve itself is inflammed, causing these sudden sharp pains.
You are correct in that this is difficult to diagnose, since there isn't a test that really diagnoses these types of pains. It is good you had the imaging tests and other tests to rule out other potential issues, but perhaps if this continues to go on, your doctor or a pain doctor can perform an injection (a nerve block - which would be some numbing medication and steroids) right in that area -- and if the pain is relieved, then you've got your diagnosis.
Perhaps something that would help inflammation, such as Advil, or perhaps a short course of steroids, like a Medrol DosePak.
Also, if these are nerve-related pains, occasionally medications like gabapentin can "dull" out the pain as well.