bd, I think I had the same problem for a while. When you say "knot up underneath" is thread catching in the bobbin area, around the 1:00 or 2:00 position? (Assuming the needle is noon.) I had to raise/remove the needle plate to see it. According to the Yahoo Slant group, Singer experimented with a lint pin, designed to prevent lint build up. It didn't work and they soon discontinued it. My Rocketeer did this on and off. I tried a "lint-pin-ectomy" that didn't help, thread still got caught in the same area. I don't think the pin was the problem, but something else (bad bobbins?) was throwing the thread path off and it was catching on the lint pin.
Eventually, it stopped doing this. I'm not really sure why. I did get some older bobbins, I found in a thrift store. And I tried Bobbin Genie washers. It still happens on rare occasions.
Maybe someone else on here can offer a more positive answer.
Tate