So... I've been having this problem the past couple of days, and it was extremely frustrating. I was doing straight line quilting, but using my free-motion foot, and trying to practice using a ruler with it. Anyway, it was always fine going one direction, and when I would switch and come back the other way, it would work for a bit then start to skip stitches, shred the thread, and break it. Last night, I finally figured out what I was doing. At some point, when lining up with the previous lines, or repositioning the rest of the sandwich, I would lift up the presser foot. In my haste (and exhausted haze after working a few 15 hour days in a row), I was so focused on lining things up, and reducing drag, that I was actually forgetting to lower the foot back down! In my trouble shooting, I'd increased bobbin tension and upper thread tension quite a bit, but it kept happening... Face palm!!! Anyway, I was able to lower bobbin and upper thread tension to normal range, and as long as I lower my presser foot, miracles happen and thread doesn't break!