I am making a doll costume out of acetate taffeta that is going to have a lot of edge stitching on it. I have no experienece sewing with taffeta. When I edge stitch in the vertical direction the fabric ripples around the stitching. How can I keep this from happening? Is it because I'm using cotton/poly thread? Is it the tension? I've tried using tearaway stabilizer but as soon as I pull it off the ripples appear. Any help would be appreciated!