The answers in the Makerbot-Operators Google-Group said that the PTFE-tube may have been to small for the thicker ABS I tried to use.
So now I detached the MK6 stepper-motors and then the hot-end and checked.
My filament was stuck fast inside the PTFE and would only come out after 220°C and lots of force was applied.
The PTFE-tube of the Thing-o-Matic is (now) 3.2mm on the upper side but only 2.99mm on the tapered business-side while my new filament is 3.06mm.
I drilled my PTFE to 3.2mm (3.0 and 3.2 are standard drill sizes while I could not find 3.1mm).
In a first 30second test it seems to extrude fine.
The filament-thickness into thick air is 0.60-0.63mm .
Time for a test-print....
On the plus side, I did manage to solve the 129°C temperature-reading issue with the Automated Build Platform.
The connector was plugged into the only socket that would accept it all the tine.
It's left 3 pin-holes should have been connected to 3 free-standing pins in the middle if the board instead.
My plunger was too tight, the drive-pulley was no longer centered because the cork-spacer between extruder and stepper was getting squashed over the first few month of operation and thus the stepper-shaft deeper in the extruder then when I mounted it the first time.
There are still issues but I can work on finding a reliable set of parameters and then calibrating skeinforge for the filament-sizes and hole-diameters resulting from these parameters.