Had a time getting a FW I had sold working properly. The lady said it just starting popping top thread. Then she said she had "messed with" the tensions. After some frustration I found the gib screw was loose enough so it caused the thread to catch when sewing fast. I'ved never had that particular screw loosen voluntarily; perhaps she "messed with" that as well. And got the tension back right; she had tinkered with both top and bobbin tension and it was some kind of a mess. Glad that is behind me.
Now to veg out the rest of the day.