I hope that you have resolved your problem. If you haven't I would try going over every tension guide very carefully looking for a tiny bit of lint, it is surprising the troubles they can cause. Carefully check your bobbin case as well. How tightly wound is your bobbin? poke your finger nail against it and be sure that it isn't rock hard (I had that happen once). Back off your top tension a bit more. Just because the stitches look pretty there, they could still be pulling too tightly.
try a smaller needle.
A picture would be very helpful if you are still having problems.