Print quality: Wavy perimeter on simple geometry  

Print quality: Wavy perimeter on simple geometry

I experience a quite frustrating issue with the side walls of a simple small box I designed and want to print reliably with a good outer finish of the walls. The object is printed upright with the open side facing upwards and the issue apears along the X- and Y-axis, though it seems more prominent on the X-axis. As you can see in the picture, wavy vertical lines appear (the box in the picture ist tilted to the side - just to avoid confusion). The unwanted lines are very small but enough to catch the light and therefore are clearly visable. Even worse: depending on the wall thickness they sometimes are not as visible when the wall is thinner, which makes it stand out even more. You can see this in the picture on the right side.

Here are the things I tried so far which did NOT work:

• Belt tension adjusted (also tried higher and lower than the prusa recommended tension)
• Different Prusa Slicer profiles (all in 0,15mm layer heigth: structural, speed and the new balanced)
• Print speed (from 120mm/s to about 200mm/s) for perimeter, outer perimeter and thin perimeter
• Number of perimeters (2-4)
• Extrusion width for perimeters
• Print outer layer first
• various fill patterns and settings
• Also did a series of tests with different wall thicknesses for the 3d file (form 1.5mm to 1.9, the recommended for 4 perimeters would be 1.7, which I tried as well, no luck though) > The box I try to print has thicker and thiner wall parts, hence the issue that the wavy lines are not always as pronounced. 

...and various combinations of the above mentioned, probably also forgetting some right now. I also did several calibrations like Extrusion multiplier, Phase Stepping, Input Shaper and more than one reset to factory defaults. 

Printer is running on latest firmware and Prusa Slicer is v2.9.3. Did have the issue with older versions though. The printer is pre-assembled.

The 3D file-format I use is *.3mf not stl

I'm now really out of ideas what else I could try and also getting pretty annoyed, honestly. Would be quite happy for any ideas how to solve the issue!