What should this clue tell me about the Z-axis wobble in my prints? (UM S5)

[libertyhouseplans 2]

There's a regularly-spaced, repeating wobble in the print surface of my prints. I thought it might be related to the movement of the lead screw and/or build plate, but I've downvoted that theory after uncovering this clue:

 

          The Z-distance of the repeat varies with the model's dimensions.

 

The attached photo shows a test cube that's 1"x1" in the bottom half. In the top half, the model narrows to 1/3 of an inch. If you can get a good view of the "rings," you'll see there are ten cycles of the wobble in the bottom half and five in the top half. You can't really tell in the photo, but the layers' horizontal dimensions appear to wax and wane ever so slightly, in sync with the ring cycle.

 

The effect runs all around the outside - not just one or two sides of the model. (In the photo, you're looking at the rear face of the model, FWIW.)

 

If it were a lead screw or build plate movement issue, the repeat of the wobble would be a consistent amount, regardless of the model's shape, right?  Instead, the repeat seems pegged to a given amount of time, or a certain distance travelled by the print head.

 

What do you think is going on here?

 

Potentially pertinent details:

* Ultimaker S5 R1, firmware 8.1.0 (latest stable).

* Print core has about 23 hours on it and has been cleaned regularly.

* Filament = ColorFabb PLA, Semi-Matte White 2.85, opened within the past week, low humidity environment.

* Spool weight: 750g. Currently about 600g into the spool; had the same problem when the spool was full.

* Feeder is clean inside. Bowden tube pre-dates my ownership, but the rear collet is new, and there's no play at either end when you tug on the tube.

* For the past few tests, I've tried @gr5's suggestion (in another post) to add a drop of machine oil to the filament every yard / hour or so. Can't say much about the results yet, since I've only printed these bitty test cubes, but it hasn't hurt!

* Layer height: 0.2 mm.

* Print (total): 23 min / 4 grams / 0.52 meters.

* Bottom half: 15 min / 3 grams / 0.36 meters.

* Top half: 8 min / 1 gram / 0.16 meters.