I am getting closer to solving this problem. I got a VIDA/DICE setup. It was a real pain to get installed and running properly, but, now that it is, I can see that it will be a very powerful tool to have for maintaining newer Volvo's.
The code from the O'Reily's scanner was "p1618 - fault in lead between aw50-52 and motronic 4.3." This meant that there was a fault stored in the TCM. Once I got the VIDA/DICE going, I was able to read the actual fault. It was the transmission input speed sensor. I was able to watch a graph of the engine speed and the signal from the transmission input speed sensor simultaneously while I was driving. When the car is shifting normally, the two speeds differ until the converter locks up. When the converter locks up, the speed of the two sensors are identical. When the car starts to have the problem, the signal from the transmission input speed sensor is "noisy" and inconsistent... sometimes nonexistent.
The fact that the sensor sends a good signal sometimes makes me think that there may be a dirty connection somewhere. I've cleaned and checked the easy connector in front of the air box. No difference. Sadly, the wires disappear into a massive harness that heads under the fuse box. I guess the next step will be to find a wiring diagram and test for continuity between the connector and the TCU. I will post what I find.

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1966 122s, 1968 142s, 1969 144s, 1979 245dl, 1989 244gl