Another update on this issue, if anyone is having the same problem.
The car's idle was OK for a couple of weeks, then it started doing it again every 10 seconds without fail. :-( It also started running quite rough!
So many sites list this problem as being caused by a variety of things, but none that I can remember listed the cause that was doing this on my car, so I will explain what I have done.
The air passage and associated sensors were all clean, all functioning normally. The gas passages were also OK; I had used a significant amount of fuel injector cleaner, so I knew the injectors were clean.
So I started thinking that the third part of the starting/running of the engine must be at fault. I took off the ignition coil pack, cleaned it thoroughly to inspect for hairline cracks or bigger ones, without any success there. The coil was visually good. So I tested the coils for continuity; all coils checked out OK. At this point, I decided to order a set of spark plug wires since I did not want to take those off, test them, and put them back, as they are somewhat of a b*tch to take off and put back on, on account of tight spaces/clips/plastic tubes. That turned out to be a great decision. The wires had over 100,000 miles on them, so I thought they should probably be replaced anyways. After spending half a day doing that job, the car runs fine - no more idle issues, no more rough engine running. I put on NGK silicone wires. The rubber OEM ones had seen better days. I didn't bother looking for any faults on them as I was panning to ditch them anyways; in fact I had to cut the three going to the passenger side of the engine because they were sandwiched between the engine and the firewall so tight there was no other way of getting them out. I routed the replacements a bit higher so they are easier to get at. These wires are not easy to replace individually as they are in sets of three with all the clips and all. So it's a good idea to make a sketch of where they plug in before taking them off. If anyone is doing theirs and gets the wiring connections messed up, let me know and I can help (made a diagram for my own future reference). Model years of the new edge style, other than 2000, may have the coil pack rotated 180 degrees adding to the confusion if you plan to use the YouTube videos. :-(