Odd. I have an ATSF 3463 3/2 Hudson that was having exactly the same problems with
my GG switches. I suspected something complicated, but after looking at the wipers
I saw that a couple of them were not touching their axles. I gently bent them and this
4-6-4 will sail slowly through multiple GG switches (old and new styles,
with no special wiring) with nary a flicker.
I know that you've been through all this, but the 4-6-4's AC common-pickup arrangements seem well-designed - and my Chapelon 3-2-1 has never had a pickup issue.
GG and Ross (I have a few) switches are pretty similar, of course. Strange.