Ribbing, banding issue i3MK2

I hope someone can help identify this issue I have with a print on the i3mk2.
When the first 1,5mm layer is build and it starts with the walls of the case, the first banding starts to appear.
Then it goes well until it reaches the cutouts on the sides, Big grooves start to appear.
On pic2 you can see the grooves are less bad on the corner, this is where the printhead starts a new layer.
Something seems to cause a shift in XY direction, the part is also a tiny little bit skewed and not fully perpandicular.
Prusa support told me to check the belts tension. The Y-belt was indeed too loose but that didn't fix the issue.
I did all the calibration stuff and I also checked the 3D model but there are no boundary edges or non manifolds.
Anyone have a clue what's going on here?

Might have to do with retraction settings or a loose drive gear on the extruder motor?

i have a I3 clone had the same problem .
i changed the threaded rods to T8's and base clamps
sorted mine out a treat But i did find it worse with white filament for some reason,getting nice new.
have a look on youtube for 3d z banding helped me