Time to start exploring outside the transmission to find reasons for failure. I had an early '90's Nissan that blew the tranmission just under 200k miles. On the highway it would buck and lurch and the converter would lock and unlock like it was possessed. Wasnt bad at first, but after a few weeks of this it got worse and worse and finally died. Tore it apart, clutch debris everywhere. Cut open the converter, the clutch was roasted.
Got a reman transmission and a new convertor. Spent all weekend installing it, finally went on a drive. Worked great....for about 20 minutes, then started doing the same thing as the blown transmission. I figured a faulty reman, but then went digging. Put in a replacement trans ECU... no change. This stupid car was about to blow up a newly reman'd trans and was likely headed toward a 3rd until I decided to keep digging. Thats when I found it.
When I had installed the new battery a few weeks before the problems started, I had to push part of the wiring harness off to the side to make way for the battery. Apparently when I did that, one of the harness plugs, that contained the convertor lockup solenoid connection, had jammed between the battery and a bracket on the shock tower and had become partially disconnected. Pushed the two plugs together.... "click". Went for a drive. Perfect. Smooth as silk. Drove it all over that afternoon, no problems at all. I was simultaneously the most happy and most PO'd person on the planet for several hours. Never had another issue and the reman lasted 200,000+ miles when I sold the car with almost 400k miles on it.
MAP sensors and throttle position sensors can also wreak havoc on transmission behavior. Penty of bad remans out there but multiple failures in the same vehicle, its time to start looking at the transmission not as the problem, but as the victim of some problem somewhere else in the car. Just my opinion.