Parents on an Autism related forum I visit occasionally report this phenomena with their kids. Sometimes it's so dramatic that they can't believe what they're seeing.
Some also report similar improvements when the child is antibiotics.
Wonder - is this why are kids are better in the summer? I always just assumed it was being able to play outside that made the difference, but makes one think.
My son's been "off" the past few days. At the doctor yesterday, his temp was 95.5.