MattR,
I tried posting this a day or two ago but there were quirks in trying to reply. Anyhow.....I had suspected your problem was the TFI (Thick Film Ignition) module. I highly recommend carrying a spare module, the right size nut driver, and the know-how to replace it. Why you might ask?
I designed that module and will try to explain it's inharent flaw.
Back in those days, Ford Motor Division supplied 2/3 of TFI modules on all Ford's assembly lines while Motorola's Automotive Division supplied the remaining 1/3. I worked for Motorola's Automotive division from 1977 to 1989. I was their Thick Film designer who exclusively laid out the Thick Film substrate circuit inside it for Motorola. The lead frame, the metal pattern (or connections) molded into the plastic housing that protrude from the inside to the outside where the connectors are, is connected to the Thick Film substrate inside in two different manners.
Ford welded wires from the lead frame stubs inside the cavity, to anvils that were reflow soldered to the substrate.
Motorola shaped the lead frame down to the substrate and soldered all connections directly.
Ford's wire-weld process required a precision setup for a reliable connection. Motorola's soldering process was very forgiving and accommodating.
At Ford, if the welder was not setup perfectly, it made welds that would break over repeated temperature changes technically called "thermal cycling". Eventually the wire welds break. The wire then makes contact when cold, but lifts when hot, causing the intermittent problem with temperature.
The problem is always a threat but in the early days, it was so unreliable that Ford recalled all their modules and installed Motorola modules exclusively until they could figure out why theirs failed. They adjusted the setting on their welders and monitored it more closely. But that process is still considered less reliable compared to Motorola's soldered process for they still are known to fail for that same reason.
I looked on-line for a picture of the anvil and wire that Ford used. No luck with a Ford, but I found a GM Thick Film Ignition module using the same welding process.
For the curious, the base white material is ceramic. The silver wiring is printed on. The black squares and top hats are printed resistors trimmed with a laser beam to tweak the resistance to within tolerance. If you look close you can see laser trimming done on the top hat resistor. The brown cubes are capacitors. The gray square in the middle with all the printed wires going under it is a flip chip IC. The power die to the left is what generates a lot of heat, requiring heat sinking.
It brings back memories.
