Well I think your problem might be spread through the computer. I have fought with a beautiful Singer 503 this evening. The thing will mostly sew nice now that I took the tension apart and rebuilt it, took out all kinds of lint and junk out of the bobbin area. BUT it will sew for a while just fine. Then all of a sudden it wants to not advance then pretty soon the thread is caught in a knot. I'm looking for burrs. Maybe Mizkaki will come along and help us solve this one.