Pfaff 1540 needle jams down in bobbin area?

Needle Jams In Bobbin Case Area.

I used quilting thread in the top and bobbin of my Pfaff 1540. It sewed ok. Then the next time the needle rotated down into the bobbin case area it jammed. The only way to free the mechanism was to remove the needle. Without a needle the mechanism goes up and down the way it should, but with a needle inserted it gets jammed into the bobbin area every time I try to sew. I have changed to a brand new needle and made sure it was seated all the way up, but it still jams. Any suggestions?