I have a Pfaff 2170 and had this same thing happen just 2 days ago. I did all the usual with the thread path, rethreading (in spite of the fact that it had just been sewing fine and started the problem mid seam). I had just, the evening before when I finished sewing, given it a thorough cleaning out so I didn't think it was a lint problem. Just to be sure, I checked and there was no visible lint. BUT just to be sure I took my brand new tiny stiff brush and pushed it in everywhere it would fit and pulled out one tiny bit of fiber that had either escaped my brush (the tiny ones like we used to use on the spouts of our percolators many years ago) the night before or had somehow entered the bobbin area while I was sewing (with a different color thread) that morning. I know my level of frustration and can certainly imagine yours. I hope you are as lucky as I was and have just the right brush/tool to get any nasty lint that may be causing your problem. You can do it; just keep trying. Get up and walk away for a few minutes if you need to but go back when you feel calm again and it may just magically get fixed with your next try.