Well instead of erasing my down-mouth'ing above.
What does work for problem systems diag'ing once you can no longer know good parts swap out?
Thermal shocking. In the summer, Cool-Cold shock with the AC at Max. Winter: Heat-Hot after overnight outside sitting with the heater/defroster on Max. Use a wall plug-in hair drier/paint stripper. (we used to use cans of R-12 refrigerant to cold shock "fast and clean", a NO-NO now . . . shhhh . . . still works with R134a).
Tap/rap/thump shock the modual and adjacent areas. Really. Was an early Ford Dura-Spark modual in-field checking recommendation along with the Freon thermal shocking.
If you can make a state in function change - IT IS AN INTERMITTENT BAD, confirmed.
You break, melt something you were too aggressive testing.
System working: wiring harness twisting, pulling; plug wiggling/pulling testing. Again any change in functioning and you've focused in on the problem area.
In fact GaryG, Ford had such problems with broken motor mounts in the front wheel drive Taurus's stressing and internal work-breaking chassis to engine harness wires that they programmed in a special wire harness wiggle test mode into their vehicle computers. I've use this test mode in Fords up through 1995 to find problems. These could run running cylinder power balance tests too. I do not know if these are applicable to yours.
Last using a very strong halogen pinpoint light and a magnifying loop: unplug and look in very carefully at all of the relevant circuits harness end plugs. Especially at the power and ground supply terminals within these multi-pin connectors. Look at the plastic plug surrounds for signs of over-heating/melting/discoloring.
At the PCM/ECU in particular since all of these terminals are the same size the clever engineering trick is to have multiple terminals as positive power in, and multiple terminal as grounding out to get enough paralleled circuit wattage capacity. Lose one branch and the others will cascade overload, over heat. Then make in-use intermittent connections.
Yeah. Chrysler/Dodge 4th gen minivans at their cooling fan controller harness end plugs!
Chrysler/Dodge 4th and 5th gen minivans at their "wave soldered" instrument clusters, + and - solder connections!
Jeeps rough road shake loosening their back of instrument cluster harness plugs!
Honda and Toyota's with the coil in cap ignitions engine heat hardening then engine vibration breaking down the to ignition engine harness!
80's early 90's full sized GM car rwd's with an inner center of dash harness to harness connector that seemed to always use load overheat and die killing half the dash power!
I've hard way experienced all of these. And more. No manufacture is innocent.
So get frustrated . . . thump on it! At least be testing, and make you feel better.
On your own stuff, you've already bought it anyhow. It is yours to break, On The Road To Better.