That's my son's score differential too - everything in the 99.9th percentile except for Processing Speed. Turns out he has poor visual processing, and as a result has a bit of ADHD tendencies except when he does something he loves.
The tester actually said he might have photographic memory despite the poor PS score because he looked at the pictures once and proceeded to fill in the blanks without having to refer again. Unfortunately, looking at each new symbol was tedious, as was writing, so he lost focus after several lines and started chatting.
I see what Mich is saying regarding ADHD. According to The Mislabeled Child, ADHD may have some causal factors such as poor auditory and visual processing. To me, the "excess extraneous muscle movement" you talked about is also another reason to have a visual check.
