I did the drain and fill thing a couple times after the problem first occurred before taking it to Aamco, and it didn't make a perceptible difference.
I don't think it requires any abnormal or heavy driving to get it to overheat, and it never overheated during the first 5 years or so that I owned it even though my driving habits didn't really change. It really seems to me that something about the transmission (or the cooler) changed and Aamco did not succeed in restoring it to its previous functionality. It seems like adding more cooling can only cover up the reason it has started overheating.
My plan at this point is to take it to a different Aamco and have them repair the rebuild under warranty. With all the other stuff I've fixed on it lately I hate for it to continue to be a POS that can't be used on a trip longer than an hour.
