Battling voltage drops when switching on lights... Options

[jcwells]

Looking for some advice or solutions to fix what I think is a result of a voltage drop issue. As you can see in my sig, I've got a '73 2.0 running SDI EFI. Had to replace the fuel pump a couple years ago, switched to a two-port NAPA unit that Clay Perrine recommended as a suitable replacement for stock.

Car runs perfectly when headlight switch is fully off, with the exception of a little bit of idle balkiness that requires me to add the slightest bit of pressure to the throttle to keep it running at stoplights. That behavior goes away as the car gets thoroughly warmed up. However, when I switch on the parking lights, I'll start to get some more pronounced stumbles as idle and low rpm. Headlights switched on result in a lot more problems at idle, low rpm, and even some stumbling at anything under 3000 rpm. I have a pressure gauge mounted in the fuel rail, but no sensor, so can only read it when standing outside the car looking into the engine bay.

With the car on jackstands, I hooked leads to both sides of the fuel pump, to measure voltage drop across the pump directly. I see that I get between 0.5 and 1.0 volts drop (from 12.3-12.5v with lights off to 11.5-11.8v with lights on. When I try to get it to idle with lights on, voltage drops below 11v, car stumbles badly, and when trying to recover I'll sometimes get a very exciting backfire for my troubles.

I've run independent grounds from every lighting ground point to a common ground bus bar connected directly to the relay board ground point, hoping that would solve the problem. No joy so far. Volt meter in the car is hooked up in the traditional, stock manner, and shows the typical response of bouncing when turn signals are on, and showing 10v when headlights are on. It never reads above 12v.

This problem has gotten worse over the years, to the point where I cannot drive the vehicle at night, since it won't reliably stay running unless I keep the revs above 2000. So, I guess I'm looking for some advice that would point me towards a way to get away from the voltage drops happening when the lights are switched on. Would running separate power leads and relays straight to the headlight/parking light assemblies do the trick?