The info provided by MAVA is golden.
From your description above it sounds like the conductor plate.
I have seen this problem twice(one being my own), both had exactly the same description as you gave and both turned out to be the conductor plate. These transmissions are very good and I have only seen problems with the conductor plate, oil leaking out through the bell housing seal or circular connector and some claim there is a regulator spring that fails. If the oil leaked out it will go to limp mode very quickly after starting, not at random times.
The random occurrences have always pointed to the conductor plate.
It's expensive, but wont bankrupt you and as you have already invested in the proper diagnostic equipment follow the route of proper diagnostics first.

Another sign of a conductor plate at the end of its life is occasional harsh shifts regardless of RPM.
The conductor plate is a piece of plastic with copper tracks on it. On the one side it connects to the circular connector for the transmission harness, then it branches out and has these pads that press against the terminals of the solenoid valves that are fixed to the transmission. They usually corrode on these contact points with the valves or the seal around the circular connector leaks pushing oil up to the shifter(never seen this myself but have seen many posts).

Good luck and let us know what you find.