87 Mazda B2600 starts then shuts off after 3 or 4 seconds

I have an 87 mazda b2600 4x4 that starts but then stops almost immediatly. It has a weber downdraft, headers and I believe that the only thing that is running off the computer is the fuel pump. I tested the voltage at the coil and it appears that after it starts the voltage drops from the initial 14 volts to about 8 volts and then the engine stops. I have traced wires looking for shorts but cannot seem to find anything. Prior to this it had engine power problems and had to downshift where I never had to before. I have also checked for negative ground connection and it appears fine.I have replaced the ignitor, the distributor, the ecu, the coil, and the ignition resistor. This seems like one of those never happens problems. PLEASE HELP ME!