Stupid question, but is it trying to fire as you let go of the key?

I always have to question how the aftermarket ignition was wired. It is not uncommon for someone to grab a 12v "run" wire and use that. The ignition, or distributor in this case, needs 12v in both the "start" and "run" key positions.

Depending on your starting issue, it could be as simple as that. I'd verify the distributor has 12v on that red wire in the start and run key position [red wire]. Disconnect one of the two small wires at the starter relay to check the "start" voltage.