I'm wondering if there's a burr on your hook or needle plate. Run your finger across the point of the hook (the thing behind the bobbin case, that goes around and has a point), and around the opening of the needle plate. If you feel a rough spot, you can polish it down with a very fine sandpaper -- about 400 gritt.

Thanks quiltedsunshine. I'll look and feel and see if I can find the parts you're talking about. It's just so odd. It does it randomly. At first I looked to see if perhaps it was always at seams. But there's no pattern to it. Some rows of quilting are just fine, and then the next might have several.