i was taught that if the problem is on top, then the blame is on the bottom. if the problem is on the bottom, then the blame is on the top. i would start by trying a tighter upper tension.
btw, i also was told that if the bobbin 'chase' is plastic as in so many new machines, then the bobbins should also be. the bobbins should be the same as the chase. i'm gradually tossing my old metal ones out and replacing them with plastic and it makes a big difference. they work a lot better. no, i'm not talking about the bobbin holder itself. that's the bobbin case.