I had a prob just like this matey

If the black switched live is fine and stayes fine when turning over then start on the green side this runs in the loom along front panel and up to ECU which it also feeds back out to the rev counter i was hoping i would find the prob in this trusty green cable but was fine

In the end i changed the dizzy and was taken from a car i had seen running with my own eyes and bolted it on and BINGO was fine and still is

The ignition system on these and fuel system is gay and belive the EFI convershion is the way forward