On any given day, I can take an IQ test that varies 30 or more points from a previous test, though I don't believe I have ever scored below normal. It does give you a rush to score in the genius range, but from a practical point of view, I know that I don't belong there except in perhaps one or two extremely narrowly defined areas.
I know my limitatons of being able to comprehend, embrace, and fully understand certain spatial or technical or mathematical concepts, nor do I have any compelling interest to try to learn to understand such things. In other areas, concepts are effortless and ridiculously easy for me while others whom I perceive to be of equal or superior intelligence to me simply can't seem to grasp them at all.
Perhaps the element of intelligence that IQ tests cannot measure are those unique natural gifts with which each of us are endowed?