Hi,
I happened to think of something this morning and wanted to share it with you, maybe something for you to check. My regular sewing machine was stitching awful and I tried adjusting the tensions (both) until I was blue in the face. I lived near a really good dealer then and took my machine in to see him. He found it right away. Seems I had a small, tiny, thread stuck in the bobbin area where I couldn't see it and when he pulled it out I was shocked. It was lodged on the inside of the bobbin case between the thread slot and where it gets hooked around before exiting the case. I also took a maintenance class there as well and he said to take an index card and slide it in the tension disks to remove any build up or loose thread. Maybe this will give you a few more options to check. Good luck Judy