I have that sometimes, but mine is much more loopy thread, with my Viking. I've figured out that the upper thread has somehow come out of the tension, when this happens. There doesn't seem to be any rhyme or reason as to when it happens either. It's very frustrating, because the stitching doesn't feel any different, until it jams from all the thread underneath. I'm not sure what the solution is, as I've had it checked when I bring my machine in for it's yearly check-up and cleaning.