sounds like ballast resistor either not fitted or wired incorectly on ignition coil.i think on most older fords that a resistive wire is fitted to coil which operates when ignition is on,when cranking a 12v feed from solenoid feeds direct to coil [which say is 9v ] which boosts spark for easier starting, then when running the resistive lead [say 3v] comes into operation "3v + 9v = 12v. on some BL vehicles it is a seperate resistor unit which you could fit.  hope this helps.
                 k
                
 November 2008