I thought I could just live with this, figuring that it was just what 3D printers do. But the more other printers I see, the more I know that not to be true. And it should be fixable.

Here is more information on the mechanics and geometry of my printer:

X and Y axises are configured identical. Motors are 200 steps/Rev. Driver configured for 16 microsteps/motor step. (3200 microsteps/rev). 18 tooth gear with a 2mm belt pitch. So for Marlin, I have both X and Y set to (200 * 16) / (18 * 2) -> 88.8888

I measured the repeat of the pattern more accurately. It is 83 ridges over 60 mm, so .723 mm/ridge.

I idea that it might be extruder related didn't really occur to me. But now thinking about it, I would have thought that the PLA would be too "wet" at the point of contact to be effected at all by tiny steps of the extruder motor. And with the ridges to precisely lined up from layer to layer, I still keep thinking it has to do with X and Y motion. If it was related to the extruder, then I'd think that there would be no reason why the ridges would line up vertically along straight walls in X and Y.

I don't think it's a resonance problem - although the parameters of the X and Y motion are the same, the mechanics are completely different, and the pattern is the same for both X and Y walls.

Another clue! If the vertical wall is 45 degrees off the X and Y axises, the spacing of the ridges expands accordingly!

Still scratching my head.