Singer 690U wraps upper thread around bobbin and freezes?

My Singer 690U has a "magic bobbin" that fills the bobbin from the sewing thread. I've had this machine since 1974 (bought it new) and have not had much trouble with it. I used to sew a lot when my kids were growing up, but now only use it occasionally. I got it out to mend something last night and it would sew a stitch or two, and then the upper thread would wrap around the bobbin--as if it were trying to thread the bobbin, but it's set for sewing, not bobbin threading--and the machine would freeze. I tried cleaning it (badly needed it!) and oiling it, but that made no difference. Any suggestions?