Would that explain the problem going away if you turn it off for a second?
Yes. when shut off the engine will then restart on open loop and run for a minute or longer before going into closed loop operation. Pre OBD-2 Fords can be a pain to diagnose even with a breakout box and a code scanner. With a breakout box and several DVOMs it was possible to monitor live data but there was no way to capture the data and very difficult to monitor while driving. I had some success using a 4 channel digital oscilloscope and a hand full of LEDs in a breakout box but OBD-2 was a great improvement.
And FWIW most sensors can operate correctly through a great deal of their range while having a narrow segment of their range defective. Throttle position sensors often fail at idle and/or at the position where normal cruise speed occurred while temperature thermistors often drop out at mid range. Static testing sensors will rarely detect a faulty one.
I have a scope and all the gear I need, but the intermittent nature of the problem is a pain, as well as observing anything while driving. When it fails, a simple key off-on (while driving) fixes it, sometimes for the rest of the trip. It seems like it runs fine COLD, then as it warms up, it sometimes starts shuddering. A Key Off/On fixes it for awhile. “Awhile” seems to roughly related to how long it’s been running. After ten minutes or so, an off/on fixes it completely until the next day. The temperature gauge has already stabilized, so I’m thinking one place where the temperature might still be climbing would be within the ECM. This is a backup vehicle that I don’t use very often. The recent test drives have been more miles than the previous six months. If I used it more, I’d buy a replacement…
Also, the shuddering is not subtle. People following me think I’m driving erratically and fall way back - doesn’t seem like the performance difference between open loop and closed loop would explain it. Similarly, if the SPOUT wire opened, the timing would just lapse to the default setting, where the engine would still run pretty well. Turning the spark or fuel off entirely seems more likely. When it’s in the shuddering mode, a little acceleration fixes it, which leads me away from a fuel problem. Maybe I’ll put SPOUT and SPIN on my scope and watch it while driving. I can put a third probe near a plug wire.
If you have a lab scope get a schematic of the computer. You can back probe the SPOUT, etc at the computer. And your lab scope likely has freeze frame.
Only test secondary ignition with an induction pickup. Spark plug voltage would likely do some serious damage to a scope.