Turns out it was the Ignition Control Module!

Well, I pulled out the wiring harness and replaced the four bad wires. I checked several others but those infamous four were the only ones with bad insulation. Since I was in there, I also pulled the Ignition Module harness and replaced the wires going to the knock sensor and the two that connect to the engine harness as they had hard insulation that had cracked. Put everything back, nice and clean. In the end it wasn't a bad job, and a good way to get to know the engine.

Once everything was back together the car fired right up, and I thought it was running great, getting 20mpg (up from the previous 15mpg), but there was an occasional hiccup. Then it got cold, and the car was back to its old tricks. So when it was running poorly I took my timing light to it and saw that when it hiccuped, the timing retarded momentarily. The only thing left was the ICM. So I swapped the one from my 88 and after tightening some contacts the problem was solved. It started fine this morning and is running great on this snowy night.

Thanks for everyone's input, and for giving me the courage to pull out the harness. Even though it wasn't the source of the problem, those four wires would have failed eventually, and it was good to get in there and clean the engine.

(now I need to get a new module for my 88)

I think you are working with your thoughts leading to components most likely to cause problems you describe.

You first worked through all the bad gasoline items that probably were there due to sitting so long. Best logically choice to do IMHO.

Let me see if I am reading this right. I will try to think this through. You tell me where I make a wrong turn!

It appears that it starts up and idles fine when its cold.
You can drive it around and it shifts up though the gears and pulls nicely until you get up around sixty mph. It is then the power band flattens out and the engine mixture acts like it is running lean or another term running out of gas. The timing could not be advancing far enough but I would think it would do that every time at lower speeds. Just make sure the nylon hose is giving vacuum to the ICU out there under the hood.

When you are at lower speeds, the lower gears reduces the load on the engine at it will pretty much rev up and pull freely. It's after it hits third or overdrive it is bogging down under more loading.

If the engine is getting a lean mixture and the plugs or wires are marginally good it's an ideal condition to develop a high speed miss. Improperly gapped plugs or just one, will cut power 25%.

If we take away that the ignition system as being in play here, we have to address the chances of lean mixture, at high speed, on cold days!
On warmer days the engine does not need to be so rich and leaner is fine!

I can think of two things that play a part in this game.

First is a cold running thermostat being number one!
Second is the ECT and wiring to the ECU. You are aware of wiring issues.
These two items will work the same right or wrong but if the weather changes their stuck doing as they do and cannot compensate.

Personally, I would want a Wahler thermostat in there, at 92 degrees centigrade, that's standard issue!

Since are into peeking at the ECU I would look and tell us if you have the 561 and what color the label is. The pink ones have been problematic. White ones are better.

While under the dash, with the battery negative terminal disconnected, pull the connector probe PIN number two and other lead to ground. You want to have resistance changes but holding between 3-400 Ohms Hot and 2300+ Ohms Cold. Since we suspect harness issues,it a good time to do a wiggle and squeeze test. If that wire is frayed, it may be touching other wires inside the wraps that may give a parallel to the resistance, through other paths, go lean or be an open the circuit and go rich.
The TPS can be checked from down there too! It's a pretty simple set up. I would have to look up the PIN numbers for you. You could check diagrams or follow wire colors if you got real good eyes.

What twists me up, is the lack of a complaint in starting? Need more symptoms for clues.
What does it do most of the time?
Easy, in twist the key from outside the car or it cranks a lot? That is, So hard, you prepare yourself to sit there.

How are the plugs running in color across the four? You should have gray but light to white means lean. White insulators with no speckles and squared off electrodes.
Any soots or oil fouling?

I will try to follow this thread and see what others say!
Phil
P.s. I just saw your post that you change the ECT. Good on ya!
I would still check it out. It does the wiring and will make sure you got the proper one in there. Some posters have been given the wrong ones!

86 has issues with biodegradable engine wire harnesses, if yours is original it is probably bad.
Dan