I haven't read all the posts -but........I used to have one. Here's a few things.
1) backing is rolled too tight -- it didn't like it so tight that something would bounce on it.
2) lint -- LINT LINT LINT -- even though you've cleaned the bobbin area -- use some compressed air and give it a shhhushhh -- more will likely come out. Get some dental floss and floss the upper tension -- lint builds up in there, too!
3) needles --- the correct needle, the needle put in correctly -- the skirt (an indented area on the needle above the eye) needs to be in the back. Make sure the needle is all the way UP into the shaft. The correct needle for the thread is important on some machines, but the Pfaff seemed to take most any thread.
4) Tension -- make sure you can pull the thread out of the needle without it breaking. If it breaks, then the upper tension is too tight. You may have to loosen it, tighten the bobbin....it's a balancing act to get them both working together.
5) Leveling -- my machine was really picky -- if the table wasn't level, then it acted snotty. If your machine rolls away from you or toward you when you let go, then you're not in balance. It should stand still when you're not pushing it around.
6) Thread -- Quite frankly, I find that with both machines I've had/have, the more expensive thread was the touchiest and easiest to break!!! Try some cheap coats and clark and see what happens. We live in a LOW humidity area. If you live in a low humidity area, put the thread in a plastic bag and spritz it with water -- just a pump or two on a spray bottle. Close it up and let it set overnight. Honestly, that DOES make a difference!! The threads will dry out and break much easier.
Good luck!