That looks like you are using the side flutes of the cutter. Maybe a cutter with only two or three flutes.

When using the side flutes, it will try to work like a screw thread against the side of the job, running up and down the material. So any up/down movement of either your quill or vice will give those results. Try locking up the quill and tighten up the gibs on the cross and topslide.

To obtain really superior results, get yourself a good quality end mill with as many flutes as possible, and razor sharp, the ones I use have at least 6 flutes. Run at high speed, conventional milling only, no climb milling, slow feed and fine cuts, plus of course, plenty of lube. If you get it right, the surface should be like a mirror.

Bogs