Heh, memories. Getting about a mile down the road on the bike before my scalp caught fire, and having to pull over and take off the helmet to tap carefully all over my head (not scratch, or there would be grease galore and bleeding and worse itching...). Fun.
What worked for me before diet (lucky me, simple Primal was a 95% fix) was essential oils - you can use them in coconut oil, or a jojoba/almond or jojoba/grapeseed blend. You'll need to experiment a bit: if the problem is an infection, tea tree + lavender + rosemary would be good, or possibly even thyme. But if it's just radically inflamed and sensitive, starting with lavender + chamomile (preferably German chamomile if you can afford it) is ideal. Make sure to get & follow advice on rates of dilution - more is not always better.
If you have time, maximise oil absorption like this:
1) Shampoo & rinse hair once
2) While it's still wet, massage the oil blend into the scalp.
3) Wrap head in warm towel and wait an hour.
4) Shampoo & rinse hair again to remove the oils. (If possible, avoid conditioner, just use apple cider vinegar &/or herbal infusions like chamomile, lavender, rosemary...)
That, and find the plainest, most innocent natural shampoo you can, or experiment with clays instead. But use all the strong anti-dandruff shampoos to clear the drains or something, and keep them away from your poor scalp.