Do you have a stitch regulator connected. After several years of putting up with this, I found it was the knob that governs the stitch length. I gave it a good tweak and haven't had problems since, so perhaps there's a loose connection in there. None of the sewing mechanics could offer any help, even the guy I bought the machine from. There may be a better regulator available now. Do let us know how you go with the owners.