Is it an original fuel pump (assembly)..? If so I would first check grounding as ant797 suggested.
Next to check is just tap the bottom of the tank - if you can see where the return line or pressure line start with some light taps with a rubber hammer along that axis.
The idea is that the fuel pump assembly sits on the bottom of the tank, so two things can occur..
1) the float arm free's up if stuck
2) any electrical connection that's gotten loose over time in the assembly settle back in place or come off - ether way you know it's where the issue is.
Note , don't bash, just tap - the plastic is old and can be a brittle by now.