Stalls with electrical loads

At my wits end with this car. It stalls sometimes just by turning on the headlights. It will stall when thrown in reverse and will stall when the fan setting is on high. I have done the following:

Checked engine vacuum at idle-19"
Swapped Carbs with a known good one of same type-Holley 600
Checked PCV valve
Ran new alternator wire through firewall same for battery feed
Cleaned the connections at the bulkhead that were not replaced
New Plugs-Wires-Cap
Replaced Pick up Coil
swapped ignition coil
Replaced Ballast
Replaced ICM
Cleaned every connection and ground I could find
Battery Voltage checked at 12.55 not running
Over 14 volts running at batt terminals
Replaced alternator-no change
Tried bypassing amp gauge

I have done some testing as shown on this forum for voltage at coil, etc., everything seemed to check out. Problem has persisted off and on for a couple years until it's to the point of not being driveable, it has gotten worse. It will only idle in gear if vacuum advance is hooked up to manifold port on carb. Timed port will cause it to stall in gear immediately. Timing is set at 14 initial. Engine runs fantastic, it just won't idle with any type of electrical load on it.

Frustrating thing is when I think it's fixed it will be fine and then all of the sudden the problem is back. In my 35 yrs of fixing cars this is the only time I truly feel defeated.

Specs: 1973 Plymouth Duster with 360 ci, 4bbl 600cfm Holley, 904 Torqueflite, RV Cam, 8:00/CR, Headers, 78amp alternator.

Somebody please chime in with some advice.