Start with easy stuff, check the battery posts/connections. Are they clean and solid. Look for corrosion. Make sure all ground straps are the same. Have had a battery cable corrode inside the insulation that caused a similar problem. Cable looked fine but when you grabbed it the insulation just collapsed in your hand, all the wire inside was basically gone. I've also run extra ground straps (my truck has had a + to - conversion done) from the battery to the frame, engine to frame, body to frame, distributor to frame. I went thru a period where it seemed all my gremlins were electrical. Maybe overkill and ground straps by themselves didn't solve my gremlins but I haven't had any problems (just cursed myself didn't I?) since fixing my distributor ground problem. Maybe it's something as simple as this.
Jeff
1964 Series 2A SW, LHD mostly stock, often runs!
1991 Range Rover Hunter