Ive been there with my 280ZX, the same problem, I have replaced wireing, relays, and all fusible links but still havent found the problem. I have checked the ignition switch and it was good according to the FSM, that is also how I did the wiring for what was involved.
Im still at a loss as this only happens when it is cold or very damp outside, during the summer months, I havent a problem with this at all, starts up everytime all the time.
I wish you luck in your search as it might also assist me. I hope you can track this gremlin down as I have been trying to for the past 2 years.