All the obvious answers have already been given, that being said sometimes it can be the design itself. Are you trying to stitch the same time when the problem occurs? Digitizing is a skill. I had an embroidery store for years so I am experienced embroiderer. I rarely if ever let someone bring in their own design. If they insisted, I would only accept the job if I stitched it out and it stitched out ok for me regarless of what their sample looked like. Same goes with designs you buy. Dakota Collectibles was consistently a headache for me. I owned the whole collection and cringed when someone wanted to look at the book. Just becuase you paid good money for the design doesn't mean it will sew nicely or nicely on your machine. My suggestion if you are stitching the same design, switch designs and see how the machine works. It might just be the problem.