Another possibility is the fuel pump check valves are cracked. At low cranking speed it is not enough to overcome the slight loss from the cracked check valves whereas at running speed it's OK. If you pull the lines into and out of the pump, and hook up a line to the outlet, try blowing backwards through the pump. If you get any air loss through the pump, then either the diaphragm is torn or the check valves are leaking.
Personally, I stay away from the ethanol fuels on both my own vintage vehicles as well as the museums, and that has helped a lot.
But the voltage one is also a possibility. On my carrier, I have a 12 volt battery which is dedicated strictly for the starter, the remainder of the vehicle is on the 6 volt system. Since there is no draw against the 6 volt system on start up, everything works vetter than original.
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