Update: I got the measuring equipment and, following Joe's instructions, the gap at the top (D) was too big. I adjusted it. I tested it. The first five tests of very fast sewing were great and on the 6th test, the thread again frayed and came unthreaded. I decided to test it on an actual quilt. The stitch was just great for the first part of the seam and then frayed again and unthreaded. I rethreaded, checked the bobbin, and tried again. This time, even on a slow speed, not only did it fray and unthread, but it unseated the bobbin again AND CURVED -- not even bent -- the needle, a Singer 16. Grrrrrr! I cleaned out and oiled the bobbin area, rethreaded, reseated bobbin, etc. This time I gave it a Schmetz 90 needle. I was aggravated by then and went off to treat my sunburn. I'll try again tomorrow.
I think I will check along the whole path for any burrs, try a different thread (previously done), and see about the difference a needle makes. I may have to play with the gaps again (thanks, Joe!). I'll just do one thing at a time.
I'm hesitant to say timing; I think it is more likely to be a bobbin area issue. Do you agree?
Why won't that darn machine work right???
Cricket