I had this trouble with my Juki when I first got it. I tried EVERYTHING! It was soooo frustrating. The thing that finally made a difference for me was Sewer's Aide. I run a little line of it along the outside of the spool of thread before I put it on the machine. I think it keeps the friction down when the machine gets going really fast. I also use bobbin genies with prewound bobbins. It doesn't seem to matter what kind of thread I use when I use this "system." I hope you find a good "fix."