Could be the ignition itself overheated and shorting across inside, or at the ignition solenoid in the 12v electric bay (streetside forwardmost) the solenoid’s high current contact’s are not completely separating when small post ignition power is removed. Built up carbon from repeated arcing can cause contact points to still barely connect and pass a small amount of inconsistent current.
Others more in the know might think the solenoid’s diode was bad, allowing feedback? Probably not.
Both the GM ignition in the dash and the solenoid are known to be problematic.
Joel