Even on an IQ test, my dd9's perceptual reasoning scores have varied wildly. At 7.5, her PRI index was at the 99th percentile, the next year at 8.5, it was at the 75th. On the CogAT, nonverbal was around the 89th or something. I have a really erratic child whose test scores seem to vary depending on her confidence level and other factors, though. Her achievement scores do the same thing -- 99th+ or high avg depending on the day.
There was a study on the OLSAT that said that it had a negative correlation with IQ for gifted kids. It is old and probably isn't available online anymore, but you can find some info about it here:
http://www.hoagiesgifted.org/tests_tell_us.htm Scroll down about half way to the section entitled "intelligence tests."
