I'll be watching this thread, as that has happened to me. Two things I've found help some - first, as mentioned, basting very very well. The second is to machine quilt with a longer stitch (on my crummy machine, I piece on a 1, but quilt on a 3.5). Also, I try to move the fabric top and bottom though together by grabbing it, not just relying on the feed rails. I do not have a quilting foot though. I think different battings are more likely to bunch than others, but I don't have enough experience to know which would be better.