SSL2013

Hello you all ! I am owner of a Pfaff 491 industrial sewing machine with the folllowing problem: when stiching, the loop thats is build by the upper thread and is normally caught by the hook sometimes isnt staying in a 90 degree angle to the hook but turns backwards so that the hook misses the loop. This results in missed stich. https://youtu.be/aKOQkTJQ338 I made a video to show the problem. In case you have any ideas I would be very happy for suggestions. greetings ! ssl2013