Intermittent slow crank when coil wire is plugged in to the dist

MSD digital 6 has a built in timing retard for this reason. The unit retards the timing 20 degrees while cranking. . As soon as it starts and reaches 800 RPM's it advances it 20 degrees. You can also do this with weights and springs if you know what you are doing . Weld up the slots from 1/2 to 3/8 outside and use one light spring to hold the advance until it starts. 1/2 -3/8 is 20 degrees . The light spring will hold the timng back until it fires up.

The kill switch mentioned above is the easiest fix. switch on after you have it cranking.