This happened finally. The car does not stall anymore.
The story: early this year it stumbled once or twice a month on the HW, then in June it stalled for the first time and since then the problem statrted to get worse.
By October I had real hard time driving: it failed to start and stalled often. I took it to the local Nissan dealer three times in June but they did not find anything. The dealer thought it was ignition: wires, plugs, distributor cap and rortor or distributor itself. I left a large amount of money there for nothing.
Another local mechanic specializing in 300ZX found that it was the fuel pump. He replaced the pump but it also did not fix the problem.
Just because the engine did not stall every day and there was no pattern in the occurence of the problem I was convinced that it was a broken wire or connector pin.
To find the problem I made an indicator of two LEDs taken from an older computer with a 4 kOm resistor ballast. I used that indocator on every possible wire in the sequence starting from the ignition lock switch and to the fuel pump harness connector. The last was the source of the problem. The connector itself is sitting under the right side of the back seat, right where I had a childseat installed. This might have caused the +12V pin to break.
For now I bridged the connector and the problem is gone. End of the story.

 

Well done..youmust be psyched to have finally discovered the problem

 

Oh yeah! I was going ***** every time I had to turn the key back and forth dozen times before the fuel pump would start.