Ok so the main issue is down direction on inspection, but do you still drift up when trying to go down? That to me still indicates a loss of pattern or at least enough to stall. I seem to remember sometimes they may just shut off the output when it goes beyond a point of no return, but cant remember if its this type. The fact that it runs ok on normal indicates that the generator is ok( ie no grounds as at full speed this would happen even more I would think) and the fact that it drives at other points and speed would indicate the inspection control is working ( assuming that your doing it from both the car top and the controller for the same result). I also thought of the load weighing but you have cleared that, so it does look like its a drive output difference. Have you tested the voltage at the motor fields, armature when going up/down?
Edit: from further thought I think you get what I call a pattern (profile generation) from the processor but it also goes into the amp for the generator field, so you could check the difference on that output when driving up and down. Also I think I remember that the load weighing should read around 80 pounds no load. Having zero may not pre load the lift. Also make sure your spring on the PVT isn't broken. Just spit balling here as it has been 15 years since I worked on a 401.
