This may be of no help at all, but I have this same machine. There are times I think my thread catches on the take up level, like there is a growth or something on it. My Pfaff dealer once showed me how to "clean" this area. It is kind of hard to explain, but it might be helpful. He showed me how to take a dollar bill and sort of clean on both sides of it. Hold the dollar bill at the top and bottom, not folded or anything. It should have one edge towards the machine and one towards you. Gently slide the dollar bill into the slot that has the take up lever and move it up and down. Do that on the other side of that lever too. Usually I get a bit of thread fuzz out.
Other than that, reinserting the bobbin and rethreading usually help too.
Hope this makes sense, and hope it helps.
Dina