6 or 12 Volt? 12 Volt should have a "Ballast Resistor" They can break down and become intermittent. 6 Volt will not have or need one.
The coil might be letting go.
I run a fuel filter between the gas tank and the fuel pump. I run another one between the fuel pump and the Carburetor. Helps keep the crud out of the pump and the Carburetor.
Sticky needle and seat will cut out under a big load(Climbing a steep hill at speed) and get better when the load is less. And last but not least, the fuel pump may be unable to keep up at sustained high loads. That should keep you busy for a while.
Start with the Condenser as suggested above. While you are in the Dizzy look at the wire from the coil and the "plastic" that the screw to the points runs through. That can crack or crumble sometimes. Then look at the Ballast Resistor(if you have one). Then do the test for the pump, and last look at the Carb. Let us know what you find.
