Photo Credit: Courtesy of 3DStockPhoto ( image)
Mine is a shot in the dark, but the fact the overextrusion is so regular let me think it is due to something rotating going around in cycles.
The fact that the pattern changes with the flow parameter make me think the culprit is the stepper motor pushing the filament (as different flow means different number of rotations for the same lenght of printed wall).
A possibility could be an irregularity in the cobbed wheel biting it the filament: a longer tooth would push more filament into the hot end, for example.
Another cause could be a problem in the actual stepper motor, in which a failure in the coils causes it to move to steps instead of one, at some point.
There are possibly other suspects in firmware settings and stepper dirivers, but that's not the area of 3D printing I am most expert in.
Much more unlikely, it could be a defect in the filament, but that's easy to test: just swap it for another one!