First of all, do you have the felt circle on the spool to reduce friction?
I have a 15-91 that I use as back-up and for straight-line quilting with a walking foot. It was doing this same thing on me Saturday. I had to switch from the large Gutterman cotton thread spools to the small ones due to thread color. Those spools are really light weight. They spin around more than the other kind in a kind of loosey-goosey way. Reducing the stitch length helped, but I think the straw idea mentioned above would work also.