I guess I will find out soon enough whether the needle is the culprit! I need to sew a quick seam down the backing, and then I'll be making the quilt sandwich. I'll use some scraps to see whether the new needle and thread solve the issue. Last night, sewing faster seemed to help fix the problem, though I shouldn't have to sew at rapid speed to have even stitches! Oh, and I measured the bobbin (I don't have the package anymore to see whether it was for my specific machine), and it did match at least in that regard to what my machine's instruction manual called for.