I know you've probably checked this. And I'm going to ask because you made no mention of it. So... just so it's out of the way.

How are your fluids?

Automatic transmissions are hydraulic creatures. 99% of the time when they're sick... it's because of a fluid problem.

If you just changed your fluids and used the right stuff, and it's still doing it, then the valve body might be clogged because fluid pressure is used to switch gears.