Thank you, everyone! So much good information on this board, it's amazing.
Interesting about the bobbin possibly being the problem. Hadn't thought of that. I've re-threaded the machine step by step from the instructions, changed my needle, and have been fiddling with the top tension. It's not an easy adjustment and I'm not sure I've got it right. I can turn the top thread tension knob round and round, and it doesn't seem to change the tension that much. Now I'm not sure I've got that set right.
I guess I'm going to have to take it in. Thanks again, everyone, for your insights and suggestions.
