[Gromit]

My sense is the answer is more likely in the very early responses to your question; particularly geezer99's

But more on the kooky side (which is my specialty) where is that purple wire going? Because if I lay a wire right up against an HEI ignition wire; I would expect that ignition wire to induce a relatively big inductive current down that purple wire. Maybe it is an optical illusion in your photo and it really isn't right up against?

Hopefully someone who really knows will correct me here and I will learn something if I am wrong.

I do think that the induced current falls by at least the square of the distance between the wires, so you could eliminate my concern my just putting a zip tie's distance between those two wires.

One strategy that has helped me is to always have one extra carb and one extra distributor rebuilt and cleaned up and ready to swap in. They are quick to change out and it can help for troubleshooting. $335 for a carb and $100 for a distributor is maybe a lot but still cheaper than seeking help at a shop.

My other thought is that a worn throttle arm bushing in the carb can cause lots of intermittent gremlins - sometimes its fine then the next minute it isn't. Just two or three thou wear on the throttle arm bushing can make the difference (I think)..