Thanks.
This post shows pretty much what I’m seeing. Der Froschkönig - Lowrider 3 in Oldenburg, Germany - #170 by Tokoloshe
It looks like the forces build up during milling and get released when the mill gets to the end of the grove.
Also the video you posted shows what I see over here.
I did a new test with lower RPM and I think I over did it a little. I have to guess, but I thing it was around 7000 rpm. And that was to low and increased the forces even more, resulting in missed steps. First in X and later on in Y too. All with a 1/8 bit and 6 mm DOC. Clearly the forces got to high even with a small endmill like that.
Before I go try different thing I’m going to get some new and hopefully better endmills.