Cricket,

I'd say just from your description that some of the broken thread or a piece of lint is stuck in your top tension. You might as well give up and take it apart and clean it thoroughly.

Gremlins are playing with our machines for sure.

Joe

No. The bobbin tension was fine. It did not need to be adjusted.

There are certainly some kind of gremlins afoot with the Singer 400 and 500 series. I usually have zero trouble with these machines.

Our 401A and 500A were both problematic when we got them. As I've used them I've de-bugged them. They are now two of my favorite machines.
My wife and I used to argue ( in fun ) over the 401, now I tell her the 401 is hers and the 500 is mine. No arguing, no fun either.

The only time I have had trouble with the 500 is with the stupid splices in the C&C thread or I threaded it wrong. Otherwise it's pretty non-picky.

Joe