I have a 1989 E350 with 460ci engine & C6 trans. It runs fine unless the parking lights or headlights are turned on. With lights on two cylinders drop out. If I give it lots of gas one of them will kick back in leaving one cylinder misfiring but back to cruise and two cylinders die. With lights off all cylinders run all the time. BTW, this van has quite a few marker lights so the light draw is somewhat higher than most vans.
I checked voltage at the "downstream" side of the main accessory breaker (mounted on chassis near battery) to ground with engine running it holds a constant 13.8v with lights on or off. With the engine off it measures 12.8v with lights off and 12.4v with lights on.