I have had the same problem with the machine stopped after a few stitches. At the bobbin is an optical eye, which measures whether there is more thread so that the machine can stop (it is brighter around the bobbin). In bright light, eg sunlight, against the machine the optical eye thinks that there is no more thread and stops. I hope this can help others with a Pfaff.
I love my Pfaff with the built in walking foot.