Fuel pump bad?

I have a 73 duster 318, 904 trans. My question is the fuel filter, clear plastic type, is sometimes almost completely empty, then you can see fuel spurting a little then nothing, and sometimes full. The car does not seem to be starving for fuel but shouldn't a mechanical pump continually pump to fill the filter and the float shut off the fuel. And when I say empty I mean not even touching the paper element. Any assistance would be appreciated.