Is the Occupy Wall Street famous 1% stable?

Occupy Wall Street has a famous slogan "we are the 99%", refering to the "fact" that 1% of the people in the US take nearly the quarter of the national income. However, this does not seem to consider that many people with extreme incomes have very variables revenues and have to borrow a lot from the private sector to finance their source of revenue, and that this revenue is unstable. Are there any studies/data confirming that the 1% that has nearly 25% of the national income is in fact nearly composed of the same people year after year? Any data that considers "permanent income" instead of "annual income"?