Missed stitches, shredded thread, bad tension, etc. - these are all common problems that every longarmer needs to learn to solve, and it takes time and patience (lots of it, sometimes). This will be the case even if you buy another machine. Since your longarm has never worked right, it's possible that it only needs some adjustments. Is there anyone in your area, perhaps another quilter, who could come over and help you trouble shoot the machine? (I would, but I'm too far away.)