Ok, I promised images so here they are!

I printed a cube at 0.05 layer height (0.4 layer width) in PLA at 200 degrees (usually print at 185, but this was a test - test failed; print quality is exactly the same.)
To give you an idea of how it printed:

It started right-back-side, and went clockwise doing 2 shells (first inner shell, then outer shell at half speed)

Left side:


Front side:


Right side:


Back side:



This left side was is very unusual (perhaps it had to do with the temp? or the layer height??) but normally my problem is (as visible in the above images too) this:


Ugly vertical waves....
BUT, they are very consistent!

Now I am aware that these are partly generated by mass suddenly stopping (inertia?) - but I've tweaked this a lot already in the onboard preferences (regarding acceleration)
THESE particular waves that you see remain even on flat lines that go on for 100+[mm]!
So be it waves / oscillations / loose belts / whatever; I want to get rid of these. I know that they 'dissapear' when I print at layer heights above 0.18[mm], but I'm looking for suitable solutions for these layer heights too.

Mjolinor, thanks for the ideas so far! I am now printing a large flat surface (at 0.25 layer height) to show you the top surface and to prove that the printer doesn't show these waves at larger layer heights.