I've had my 6600 for over 2 years now and not a single problem ... I love mine ! Best machine I've ever had, and I've owned both Pfaffs and Vikings.
I've had no thread or unthreading problems, but I've always used good quality threads. I've used Gutermann and Mettler, no problems, but I use Aurofil most now and Sulky blendables etc. Don't recommend Coats & Clark thread.
Did you buy it from a dealer ? Your dealer needs to go over it if you're having continued thread problems ...
Last edited by TexasGurl; 02-05-2013 at 08:37 AM.