I have a Designer I and when that happened to me, I kept test stitching and lowering top tension. I finally had to turn the screw in the bobbin a tiny, tiny bit to get the thread more balanced. I encourage you to test out the settings. Did you treat your sweatshirt as a knit? The Embroidery Library has some good tutorials on how to stitch out on sweatshirts. Just a thought. Good luck.