From what I've heard and read many times on this board, the most likely thing to make a FW lock up is some thread tangled up in the bobbin case. I know my FW doesn't like a single thread wrapped around in the bobbin case. I am no expert, but I can tell you who is -- Miriam, Joe Miller, SteveH and Macybaby -- these folks are very knowledgeable about the machines.
Jeanette