From 2J's Automotive:
"How can you tell if the overdrive is the problem? There are a couple of red flags you should watch out for that can give you a good idea of what could be the problem.
- Shifting in and out of gear
- Shifting out of overdrive on its own
- Faulty transmission temperature sensor
- Low fluid levels
- A bad connection in the transmission
- A bad cooler check valve
If one or more of these things are the problem then your overdrive may need to be looked at. Often times small errors can be fixed by cleaning connections."
p.s.
Bear in mind that I'm grabbing at straws on this part...but being a syncro' / wonder if there is an overdrive issue involved, or.... *
I had an old 150F years ago that was doing all kinds of weird stuff as and I couldn't quite nail down the issue - finally I have up and bought a new transmission (certainly nOt tHe wAy tO gO!)....long story/$hort-
After crawling underneath again, to begin the removal procedure, I noticed a split in the vacuum tube going to the shifting solenoid..... *
~ cutting 1/4 " off the end of the vacuum line and placing it back on the shifting solenoid fixed the problem. 'Course I still got stuck with a $150 restocking fee on the transmission I just bought - BUT, I didn't have to do anything after that. It was a pocket knife & slice off and replace vacuum line 1 minute repair.
That wordy response being said, have you checked the shifting solenoid¿? I think Imp is more on top of this than I am, but just offering my 2¢ in the hopes of helping.