I didn't read any of the other's answers.....but it set off an alarm for me.
Reminded me of my mother's sewing machine years ago.... She battled that dang thing for YEARS (caused me to not sew for years, too). Finally, a different repairman and BINGO...he used a magnifying glass and looked into the area where the needle goes down.....found a tiny DING in the metal...that little burr was fraying the thread!! Hope this helps!