I've had problems with dandruff, and an irritated, itchy scalp for years, especially since I've lived in a more humid climate (Florida). I recently tried apple cider vinegar and hydrogen peroxide, which helped a little, but not that much. I did notice recently, however, after coloring my hair with henna, my scalp felt much better, not itchy and irritated at all.
A little internet research suggested that henna is in fact a treatment for dandruff, lice, and other scalp issues. The only downside to this would be the fact that it's a dye of sorts, so you'd have to be willing to go with that. Worked great for my purposes, however, and I wasn't even anticipating it helping my scalp issues!