The symptoms you have are classic stuck valves, governor or valve body. To service the governor the tail must be removed.

This is what I would do if the car was my shop, check the level and condition of the fluid if clean with no burnt smell; I would drive it until it reaches operating temperature, then do a road test to check for any slippage, kick down up shifts and down shifts. If the transmission performs to my satisfaction, I would flush it and add a friction modifier/cleaner, Lube Guard.

Don’t believe the Flush misinformation in cyber space, most of it is BS, if a transmission fails after a flush it would have fail with out one.