my dd7 has been tested three times at age 3.5 , 5, and 7. His FSIQ went up 56 points total in that time period. (he started out in disabled range and ended up in gifted range). So i believe these tests are unstable at young ages, especially if there are any other factors involved such as speech delay or slow social development (because they won't feel compelled to please the tested).
irene