Going to repost this here:
Well, take it for what it's worth, but the general consensus I got from test-takers in September was that they were a few points below what the AAMC practice tests had indicated, while most people in August or July seemed to be closer. Mine was by about 3-4 points also, though, besides bragging rights, I don't think it will make too big of a difference for me in the application process. I imagine it was simply a case of a lot of people spending their entire summers studying, making the curve brutal.
If I had to retake, I'd go for early-mid August. Right around the time the PR and Kaplan classes end, and most of the people doing their first write try their hand at the test. You have no idea how many people in my class, who would have benefited quite a bit from more study time, told me they were writing at the end of the course so they could still have a month of summer. Of course, this is pure conjecture and hearsay on my part and I have no statistics to back it all up, so salt heavily.