In my experience, sputtering at higher speeds is usually due to a fuel supply problem. The fact that pumping the gas kept it going is some confirmation. In my 69 Dart slant, it was due to the hose on the suction side of the fuel pump getting soft from ethanol and collapsing under suction. That was 1991 on a cross-country trip when I ran into some of the first 10% ethanol gas. Insure all your fuel hose is the later "fuel injection" type.
A weak ignition usually shows itself at low engine speeds, with high throttle, like going up a hill or on-ramp. That is where cylinder pressures are highest, which is hardest to throw a spark.