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I like the wires and plugs theories; the problem really did start after I replaced them.
I used Motorcraft SP500 plugs and WR5791 wires. I swapped one wire at a time, so I don't think I messed up the firing order - besides it runs fine above idle.
I had already re-seated the wires onto the plug tops and into the coil, figuring that was a likely culprit. No joy.
Now I'll check for blue arc in dark - I've seen that on other cars back in the day, and would recognize it.
I'll also double check the plug gap. (I remember that the plugs in the box were gapped wider than spec and I reduced the gap.)
I'll pull a plug or two and check that the gap is correct - in case I read my gauge wrong on the day I installed.
Cleaning the IAC can't hurt - but I'll try the wires and gap checks first.
Great advice all. Will post follow-up.
Thanks!