The other situation could be a bad valve body. Not specifically from wear, but from bad gaskets between the two halves or worse yet, warpage. I became aware of that issue when a friend had a Turbo 400 rebuilt with many high performance parts. He was not ready to use it so he sat it on the floor in his garage. About a year later, he put it in and whenever it tried to shift into high gear, it'd start locking down. The fix was another valve body that was good. Not sure why sitting on the concrete floor might be different than being in a vehicle, but that's what more than one trans shop told me, even though it was full of fluid.
In any event, the gasket between the valve body halves is usually some sort of paper derivative. It can deteriorate. If the fluid doesn't go where it's supposed to when it's supposed to, whether from valve body gasket leaks valve body halves warping or sticking shuttle valves, the trans isn't going to act right anyway.
Not to mention that a trans with lots of miles on it probably does need some refreshing internally. Sometimes, some additives or using motor oil (20W, as mentioned in the Chrysler Police Car book) might buy some time, but we all know where it's all going to end up eventually. Just make sure that all items used or replaced (bands, frictions, etc.) are of at least OEM spec quality.
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