Low voltage at coil - after trying to start the motor?

I could use some help here. I have a Pertronix III and was using a stock coil for awhile without issue. The resistor wire was bypassed at the ignition switch.

I switched to a Flamethrower coil thinking it was simple upgrade to a very old coil. When I started the car for the first time, it ran for about a minute and then stopped. When I attempted to restart it, it fired off but never started. I immediately suspected the coil because it was the only thing that has changed. I checked the ignition wire voltage at the coil with the key on and it read about 7 volts. I jumpered the pos+ coil terminal to the battery and the car started/ran perfectly.

• I removed the pos ignition wire and check it again with voltmeter (key on) and it reads 12v.
• I put it back onto the coil, re-check the voltage and it is still 12v.
• I try starting the car - doesn't start. Checked ignition wire voltage again before shutting key off and it is about 7v.
• I remove the ignition wire from the coil and test it - and it reads 12v.
• I hold the ignition wire and positive distributor wire together, off the coil, and it measures 7v.
• Aftermarket tach wire was removed early on during the troubleshooting

Obviously something in distributor (circuit) is pulling the voltage down to a "no run" situation but only when using the ignition wire - because, as I pointed out above, when I jumpered the coil to the battery, it ran fine. By jumpering the coil successfully, is this still pointing to the ignition wire - event though I can read 12v at different times? Maybe a crap connection/splice that only presents when there is a large draw on it - but it didn't do it for the original coil? Do these coils put more demand on the feed side of the circuit?

Can someone help me by explaining what is possibly happening and why?

Unknowing - thank you for that follow up and I'm glad you found a solution. I thought about using the relay but I ultimately resolved it by running a completely new wire from the ign switch to the coil. I wound up having to replace the switch and pigtail anyway so it made it easier to do. My learning curve was extended because I refused to believe the printed Pertronix instruction that I really needed a full 12v. Shame on me.

Thanks again!

Geoff