After years of trouble free use, this is what greets me on the underside of anything I sew with my CB 4500. Using the same thread I always have (207 top and bottom). Top stitch is fine. Does not seem to matter what I sew the result is the same. Have checked and double checked the thread tension top and bottom and it seems right (did not change that either before this started). Did some research on this and have checked the items mentioned, still the same. I do notice when I go into reverse there seems to be a "clunk" sound and the upper thread appears to pull. Stumped, all helpful thoughts welcome.