Sounds like the issue I had last year. I started by replacing the starter relay ([ http://twinturbo.net/nissan/300zx/tech/starterrelay/starterrelay.html ]), but that didn't fix my issue. I tried all sorts of things like replacing the clutch switch to even replacing the ignition wiring. Turns out it was just the ignition tumbler itself being gummed up. Squirted a bunch of electrical contact cleaner up into the key slot, and black and grey crap came pouring out in the stream. Car fired right up and has been great since.