electrical blackout woes

I no longer own an A-body (had a '69 273 Dart) but now run a '72 W200 plow truck with 318 and 727. Everything is basically un-molested original Mopar except for a homemade HEI conversion (many thanks to this forum). After experiencing occasional complete blackouts of the wiring harness, I now run a dedicated ignition circuit fed by a fused toggle switch right off the relay post. The starter is still cranked via the original key switch on the column.
The other night, I plowing out my drive and neighbors and on the last nudge she blacked out completely. Kept running but no lights, heater, or anything. Once this happens the starter will only work by screwdriver jumping the relay on the inner fender. I parked it for the night and by morning everything worked fine again.

I have replaced all chassis/engine/body grounds thinking that might be the problem but no luck. Does this sound like the old AMP gauge issue? A bum key switch? Where do I test for key switch current?
Thanks