It could be a poor fit between the y axis cable and the platform. I had this problem. For an example image and solution see the thread on y axis backlash where this was discussed about 2 weeks ago. Try this: with the motors on, can you move the platform with light pressure? if so, that is the issue, that movement is reflected in every other pass when roughing in the x direction. That play allows the part to move into or away from the cutter enough to make the ridges. link here Backlash on y axis?