Im very thankfull for your quick responses. I can assure you, the backing is smooth and the pen was well attached. I used a pen so the wobble would show more. When using a marker they arent as good to see. Also, im using stainless rails. They are not rough or dirty.
Like i said earlyer, even when i completely remove the z-axis, the whobble is still there.
Im thinking more in the direction of a control problem?