Pfaff Sewing Machine Jamming and Not Stitching?

My Pfaff sewing machine won't sew more than a few stitches then it halts and does run, but does not stitch. I have changed the bobbin, rethreaded, and cleaned my machine and it starts to sew a few stitches, jams, and continues with no stitches. What am I doing wrong? It worked yesterday.