1974 Chevy Nova Car feels like it has no power and won't go

I have a 74 nova hatchback and I am having problems with it when driving it. The car starts up strong and idels good, but the problem comes when you are driving the car. The car will get up to about 30 mph and then it loses all power and then you have to pump the gas to get it past 30. Once you do that, the car will get up to about 50 or 55 mph. I can't get it past that. I have called mechanics in my area and they are not sure what it might be. They say it could be the fuel pump, fuel line, or even the carburetor. I would really appreciate the help