Update - I finally gave up and removed the transmission and took it to a local transmission shop. He called me two days later and exclaimed that it was "like a brand new transmission inside" and wanted further details of how it behaved. He finally discovered that the bolts in the governor were merely finger tight which allowed fluid to leak out of it and even more so after it got hot and less viscous. He tightened everything correctly, I reinstalled it in the car and now it behaves correctly. Case closed.