Maybe you just need to thoroughly clean the bobbin area, removing the bobbin casing and cleaning with a brush. Then rethread the bobbin and from the top, retread to your needle as well. It seems whenever I have an issue, if I just stop and clean EVERYTHING it makes such a difference.
Just read your notice re: Coats and Clark. I have found it frays and also creates so much more lint than anything else. I no longer use it.