If you are running test scraps with no problems it is probably the fabric and not the machine. I had a similar thing happen. I tried everything with the machine, rethread needle, tension, you name it. Finally I realized that the only time I was having the problem was when I was stitching on the blue batik fabric. When I used a test scrap of that sure enough, skipped stitches. When I tried a test of another fabric, also a batik, beautiful no problem stitches. Since I was not using much of the blue batik in my project I adjusted the tension and got it to work. If it had a been a big part of my project I would have switched fabrics. All the fabrics I was using had been bought at the same time, same store, just different colors. Just a thought that this could be your problem as well.