Solved!
I believe I had various small vacuum leaks, perhaps some less than pristine injectors and a dirty throttle body that contributed to the inconsistent idle issue but were not the primary cause of the problem. After weeks of not having any error codes to guide the way, today after replacing the alternator (old one works just fine, voltages are back to normal), the ECU finally detected a problem and threw a misfire code for cylinder #8. The plugs are new, the fuel injectors are new, so that leaves the ignition coil. A local parts store had them in stock, and I just swapped in the new coil and boom -- problem fixed. Rock solid idle, no hunting or oscillation.

The lesson here for me is the ECU can fail to notice misfires.

So... I ended up do a lot of maintenance work that probably was not fully necessary to solve the idle issue. I am glad, for example, that I did not go ahead with the O2 sensor replacement. But work that needed to be done soon or later -- valve cover gasket, injector seals, injectors, intake manifold seals and injectors, knock sensors, valley coolant hose, MAP sensor -- is now done.

Thanks all for the various suggestions along the way.

 

Glad you got it sorted out.
As for the other work that seemed unnecessary: it is a Land Rover, it would have needed it sooner rather than....... sooner.

Jeff