Battery cables and grounds.
At the neutral safety relay, disconnect the spade terminal so you can test the voltage at the ignition coil without cranking the engine. You will need to have someone hold the key in the start position while you check the voltage. You should have 11+ volts. Reconnect the wire at the relay, and re-check while cranking. If it is 7 volts or less you have too much resistance (voltage drop) in the battery cables.
Also adding additional ground wires from the ECU to the battery, and from the battery to the chassis helps.