Some, most, or maybe all the aftermarket harnesses don't have the Ign. 2 circuit. So, if running a ballast, the ignition isn't getting a full 12 volts while cranking. If this is indeed the case, then get a 12 volt coil that doesn't require the ballast.
That will not fix it. The "run" wire goes dead during cranking. You either have to run the wire separately yourself from the IGN2 switch contact to the coil +, or else jumper the two switch terminals together and then run an ignition with no ballast
