I cracked the problem. On these cars the current passes from the pickup wires through the truck screws.
However, the pickup wires are loosely coiled around the screws, so the screws need to be in pretty tight to make contact consistently. But when you screw it in tight enough to compress the pickup wire coil, that presses the top of the truck against the flat bolster
Therefore, the truck becomes rigid and loses the ability to "wobble." Its wheels cannot stay in contact when starting up a grade or going through curves.
My solution: Don't use the screw to carry current which allows you to back the truck screw out. This gives the truck enough degrees of freedom for the wheels to stay in contact with the rails. For power to the light regulator, I drilled holes in the chassis floor and truck so I could feed a superflex wire through and solder it to the pickup wires.
Working beautifully now, ie no flicker. Now to do the same to the other three cars in this set.
