To answer some of your questions - Yes I am performing a fully automatic calibration with an Agilent 3458A and a Voltech calibration fixture. Same meter & fixture I always use.
Are you disconnecting the BNC leads while performing the calibration/verification? I can’t remember seeing any BNC connections. Not sure if we've ever done this, could you elaborate?
I went back and forth between the ATi testers and the AT3600 testers without changing any setups or connections and both ATI testers would not pass while the AT3600 testers passed without any problems. Any other thoughts?