I have a 6600 and the only problems I have had involve slight adjustments with tension. In my experience,when a dealer blames the thread they are blowing hot air up your skirt. My guess is that there's a small issue with your discs...perhaps a small bit of lint causing trouble. I believe somewhere on this site (or on the Yahoo Janome boards) people mentioned this same issue and offered a solution to try. There is also a helpful board on the Janome site itself where people post about problems, etc.. Good luck.
BTW, I have used YLI in my machine and it worked great. Something odd is afoot.
Last edited by Ladyjanedoe; 01-06-2012 at 10:53 AM.