Thank you for responding.

re "...Or before that replace the cat, which I think you haven't done yet...."

No, I haven't yet. I asked that on my first post, but someone mentioned that since the CO levels were very low, it seemed that the cat was working, and I've sort of forgotten about that possibility in subsequent posts.
But maybe I should recheck that -- maybe I'll take my IR thermometer and check the cat's temperature -- if it's working, I'd expect that it should be much hotter than the pipe in front of it, right?

Is it possible that the cat is still good for one gas, e.g. CO, and not for another, HC?

I'll post back after I make a temperature measurement.

re "...Maybe it's not the car. Are you using the same shop to test? Maybe defective test equipment?..."

Funny you should mention that ... I thought so, too, and for this last test on Friday (which showed slightly lower "slow idle" HC levels), I did take it to another station about 20 miles farther way. But the "fast idle" level was the same.
And my plug wires are only two years old Bougicord-brand. But I'll check them along their length when it gets dark -- easier to see arcing in pitch dark (some years ago, I found the coil wire that way, shorting against the power steering that it laid against).

And thank you again.