I have the same problem of scalp itch. Do you also have a buildup of oily "stuff" that you can take off your scalp if scratched with a fingernail? It has a yeasty odor too, in my case.
I believe the root cause is candida. Humaworm has helped me greatly as has a relatively strict diet. I've nearly eliminated all grains from my diet, but have the occasional bowl of oatmeal or brown rice. I eat lots of lean meats, as well as loads of raw and cooked vegetables. Also I get lots of good fat which helps to keep my skin/scalp oiled from the inside.
I've noticed as soon as I get a craving for carbs and eat a bowl of cereal and then some bread or something, my scalp itches lots within minutes. Sugar has this effect as well, but not as severely.
Try eliminating carbs such as bread, bagels, cereals etc... completely for a few days and see if that helps. It's hard to do at first but becomes easier.