If tension is really the issue i’ve got a great solution. Fill-tec makes a prewound bobbin called magna glide. Instead of a normal metal bobbin, its thread wrapped around a magnetic core. The magnetic core keeps the bobbin from rotating at different speeds as the thread runs out. Having variated tension is something that long armers can suffer from. Using good thread, sewing on good fabric, using good batting and changing your needle will help as well, but i cannot tell you how excited i was when these bobbins actually worked for me!