Yes, brain fog, my doctor (an autism specialist, for what that's worth) says it's because of certain things that cause an "opiate effect" on the autistic brain. Those certain things are gluten, dairy, and benzodiazapines. I agree with the latter two; when I cut dairy out of my breakfast, I stopped having these kinds of days, ditto with the benzos. Gluten, I have no idea, have not verified it personally, but she's right about the other two so I don't see why not gluten too.
If you're more into chemical solutions, try N-acetyl Cystine. Yes, the same doctor recommends that at a dose of 1200mg 2x daily to prevent autistic brain fog. Haven't had one of the days you describe ever since I started taking it, so I guess I can't say it doesn't work, though I tend to be skeptical of the promises made by dietary supplements/homeopathics/herbals.