This is a very old thread, but in my search I came across it . I had this problem - none or very little dandruff (which appeared to be from scratching); no shampoo or even a prescription fungicide seemed to have any effect; the itching was always present but extremely bad at night; an allergy to any detergent or bath product was ruled out. I tried several evenings of a strong (and super smelly) tea tree oil solution, T-gel so strong that it made my scalp peel, a lotion/scrub for poison oak, and of course, the prescription fungicide shampoo, and none made any difference (except to wreck my hair).
It turned out not to be from anything outside of the scalp at all. After being diagnosed for a seemingly unrelated problem, I was prescribed Atenol - a beta blocker that reduces high heart rate (mine had been constantly over 100 at rest) and lowers blood pressure. Within about two hours of taking the first one, my scalp calmed down, and from the next day onward the problem has gone away. I'm not sure if it will come back when I run out. It was the only medicine I was taking for several weeks, so I am very sure that is what made it stop.
You can check your heart rate and see if maybe yours is high by downloading a free heart rate app onto your smart phone (Azumio makes one). It isn't supposed to be over 60 most of the time at rest (when you're sitting calmly in a chair in the evening reading a book for instance).
I haven't seen anything other posts about Atenol helping this problem, so I'm posting it here because for me the relief was so fast and so dramatic. Good luck to all.