i had a new Schmetz topstitch needle in the machine, and i replaced it with another new one during the process of trying to figure out what was wrong. I don't usually use a topstitch needle and I actually wondered if it was part of the problem, but had read to use a topstitch one for FMQ, so left it.
I was thinking more about this - the 503a just not being good for FMQ shouldn't explain all the issue. Even when I had the walking foot on it was breaking. I may try it with regular thread in the bobbin - except that it was always the top thread that broke. the sewing machine shop person did clean between the tension disks with alchohol and a bit of fabric, just in case there was dirt in there.