I have a Pfaff tipmatic 6122. I recently ran into a problem where the machine behaved as if the bobbin tension was set too light. Thread on the underside of the fabric was much too loose when sewing many layers of fabric. The normal tension adjustments and lubrication did not solve this problem. It turned out that layers of thread had wrapped themselves around a hidden part of the transport.
This is what the internals look like with the plate removed. Plate removal is described in the Pfaff manual, but is basically just a matter of prying up the edges with a screwdriver.
A closeup of the problem area. The red circle indicates where the thread buildup occurred.
Removing the thread solved the problem. Sewing as good as new!