I have a '78 280z with a weird problem, sometimes the car runs normally but suddently dies and when I try to re-start the engine again the spark dissapears, then the spark is there again and the engine is able to start, and then it happens again and again, no spark and I can't find where the problem is taking place, I already have changed the coil, wires, spark plugs, rotor and cap and nothing seems to work, but also I have notice something, when I check the negative and positive wires that connect the coil, both positive and negative have a 12v electrical current. I don't know if this may be normal, I will appreciate any help, thanks.