We had the same problem on our Pfaff. We think that we get lint balls or thread in the tension discs. Try loosening the tension as much as you can, then take dental floss to the discs and see if you dislodge anything. I have taken mine completely apart before, but if you do that be prepared to pay VERY CAREFUL attention to all the parts and how they fit together.
I hope you get your problem fixed. I understand how frustrating that is.
Darren