I think we need a bit more information or even a photo of the stitch you are talking about. Helpful information would include....what are you trying to stitch? I am guessing you are trying to free motion stitch? what kind of threads are you using in the top and bobbin? Sounds like you have tried different combinations. Are you using a batting...what kind....sounds like the stitch line looks like it is one stitch without separations made by the bobbin thread? If you getting "railroading" on top...it usually is a tension problem as the bobbin thread is not tight enough to pull the top thread down into the quilt sandwich...try using two different colors of threads...one in the bobbin and one in the needle and see if the bobbin thread is coming up to the top on a practice piece. Are you using the bernina stitch regulator...what needle?