This. Surging is usually related to a fuel delivery problem. I had it happen when my later year (70+) tank venting system wasn't hooked up properly. Pressure would quickly build in the tank and cause surging.
To rule out a fuel tank pressure issue try driving without the gas cap and see if it goes away.
If not you have several places to look:
pickup screen on the sending unit
crap in the tank
clogged fuel filter
faulty fuel pump
clogged carb jets/crap in carb/incorrect float height
if all of those fail to show any results you may need to increase the jetting on the carb at cruise, or metering rods depending on the carb