miriam
I had one do the same thing, not a 241. Found out that the point of the hook had a slight burr on it, you could only feel it with you finger nail as you move you finger from the back toward the point. When it hung up, it would pull the top thread real fast and make a rats nest in the bobbin area. I took some wore out 400 grit emery paper and removed the burr and it works good now. One thing that will indicate a burr is marks on the foot needle hole like a bent needle may have hit the hook. Here's is a link for a service manual.
http://parts.singerco.com/IPinstManuals/241.pdf