I don't have a longarm, but I do have a great machine that will do what you're describing if the upper thread is not properly in the tension. It's terribly frustrating because it looks great on top and you're smiling at the lovely stitches until the bobbin runs out you turn it over and &$*%#^@$!!!! I have another machine that will occasionally do the same thing for the same reason. Probably you did think of that, but I'm just mentioning this because I've learned that the first thing to try before adjusting the tension is re-thread, maybe more than once. I don't appreciate the fact that on these newer machines the tension mechanism is hidden from view where I can't tell if there's piece of lint or thread caught in it, and couldn't get at it if I could see it!