[Tom Goldman]

Your issue is not uncommon amongst EFI cars .
I have a lot of EFI stockers come thru the shop and it is something I've seen happen on several older built cars .
I am defiantly a proponent of dual batteries , the reserve they offer comes in handy when going rounds with minimal time to charge the batteries . ... . Running an alternator is a must with any EFI car to keep the voltage above 12.5 v.
Two areas that are overlooked are grounding and master disconnect switch .
I see many cars with no ground wire between the frame and engine or battery grounds bolted to the sheetmetal floor.
I have seen 2 cars this year with problems due to disconnect switches with high resistance /voltage drop issues. ….look at the amp rating of the most common disconnect switches with 3/8" studs ,most are only rated for 125-150 amps intermittent {starting} current . while the heavy duty ones from Moroso or Flaming river are rated for 1000 amps or higher intermittent current.
One last thought regards the wiring of the ECU itself . Holley in particular is adamant about having the ECU connected directly to the battery with no interruptions.