Do you use barettes or hair clips to pull your hair into ponytails often? Maybe (coupled with your sensitive/dry skin) the stress on your hair follicles from too-tight hair ties or ponytails might be causing scalp irritation. (Just a guess--I know lots of girls who'd get chronic dandruff and itchy scalp because they'd always wear their hair in ponytails, especially when put up overnight.)
That could be a consideration if you eliminate the possibility of cleansing agent irritation/allergy.
That might be it! I always wear it up for work. I'll try wearing it down for awhile and see if that helps. In fact, now that I think about it, that's probably it because I didn't notice it itching much until I started pulling it back each day. <3 Yay Schutz! I really hope that's it, that would be such an easy fix. And I wouldn't have to put vinegar in my hair! :D
Edit: And I'll try to cut back on the washings, too. I wash every other day right now, so I'll try to stretch it out to two days between washes, and if that goes well, three days between washes. I already use my shower cap often, using it more often wouldn't hurt me and it might be better for the hair. ^^