i'm on adderall.
I had a sort of itchy scalp thing before i started adderall. it only itches when I get over-heated and sweat. Things I've read make me wonder if it is a kind of fungi infection, as itchy head issues are apparently often some kind of fungi when they are not lice and do not exhibit sores and such (i have no sores or lice...i'm assuming you checked for these already as well). and the lower immune system thing characteristic of people with autism spectrum disorders makes this a higher likelihood.
but since starting on adderall it has become much worse. it was gone when i lived in a colder climate. now i've been in a hot climate for two years. last summer i wasn't on adderall, exercised a lot, sweated a lot, and had very little itch. this summer its been pretty terrible (like, i want to take a knife to my own head terrible) and I haven't even been exercising very strenuously as I did before.
this reminds me that i have to ask my doctor about this. if it is fungal, then it could be difficult to get rid of without endangerment of its "host", as fungi are apparently very similar to humans in some fundamental ways, so the things that could kill fungi could also do damage to the person
i also read that lymph nodes can swell and become itchy, but I disregarded that for myself, because the itch is over a large area on the scalp and not in fleshy lymph-node areas like upper neck.