the machine is now in the shop. as i said above, i disassembled the entire bobbin structure, cleaned it, and reassembled it. i actually took it apart & put it back together three times, testing each time. still the same problem - the thread gets caught under the bobbin tension spring arm. I opened up the whole machine, just to make sure there wasn't a hidden problem. the gentleman at the repair shop suspects i may have a broken or missing bobbin gear.

when i painstakingly examined the photographs in my manual (used a magnifying glass), comparing them against the bobbin machinery, i could not see anything awry, but then this gentleman's business is repairing sewing machines. i am on pins & needles (pun sort of intended) waiting to hear if he can save my baby.