I had all kinds of weird starting/running problems with my fresh 383 in my Challenger. It was a complete resto. The ECU can cause no end of grief. At first it was a bad box. Then a bad ground at the ECU/firewall (I eventually ran a seperate ground wire to the ECU.) The standard orange and chrome boxes are Chinese junk, the transistors inside are slow and do not put out the output necessary to let the coil reliably do its thing. Even if this isn't the problem, you would probably save yourself some grief by using an FBO ecu/coil/ballast resistor combo.