The problem with my SInger 9960 is...other than VERY annoying...the top threading feature. I thread it carefully, so I think, but if it isn't caught just right or whatever goes on inside there...my stitch line will look similar to what you show here. I rethread, sometimes twice...and it will then sew fine. So...rethread it. On mine...it is enclosed and you can't see a problem and you can't half way unthread and re thread..you have to take it all the way out. Sometimes I have noticed that the spool is jammed...like with those little slits in the cap of the spool to hold the thread end? The thread will be hung up on that. Other than that...I dont' know what is causing it, other than if I just unthread and rethread...it will sew fine.