I had that a few months ago. It must have been caused by dandruff, because what cured it was a bottle of anti-dandruff shampoo from Tesco. I had first tried coal-tar shampoo, which is the recommended answer, but that was too strong and made my hair go stringy and dull. The shampoo from Tesco isn't highly medicated and doesn't smell medicated. There is conditioner in it, which leaves the hair soft.
At the same time, I had developed itchy patches under my breasts, where skin was against skin. Reasoning that they might be from the same source, I used the shampoo as a shower soap for the itchy areas (I wash my fairly short hair in the shower, so rinsing it off well in all areas was no trouble) It took about four or five daily showers to get rid of it completely, but it all stopped itching after the first wash. I now keep the bottle of anti-dandruff shampoo in the shower, ready for any future attack.