Joe,
I didn't mean to come across as being snappy, but I do not actually own a cloak of invisibility. I originally stated that I did not know why dropping the needle worked, but that it changed something. Years ago, I did alterations, and learned to work with what I had when something different came in, and some fabrics just make your machine act screwy.
When I looked up your bad seed 120-491 machine, I found that on the Vintage Kenmore Yahoo group, that it was made in 1954. Also,
http://sewingmachine221sale.bizland....re/page44.html about 3 down, shows compatability to your machine.
Melinda