I had something similar happen with my Kenmore 117.841. I thought perhaps it was the timing so I ordered the service manual for instructions. While I waited for it to arrive, every day I'd play a little more - unthread and re-thread, change the needle, clean the bobbin area - etc. Mind you, I'd done all of these things many many times. But what's odd is by the time the manual arrived (about a week) all of a sudden my machine was working again. It's been many months now and it's never happened again.
I still am not sure which of the things that I did were the ones that fixed it. That bugs me.