The pump may have filled with rust. I don't know if you can clear it. TTHe new aftermarket ones seem sealed. You may have to clean the tank and replace the pickup filter. There's a baffel box around the pickup filter. The filter is accessed through the large plug in the bottom of the tank. The gauge sender opening is outside the baffle box so it would be hard to flush out the baffel box from there. You may be able to snake an air hose over through the baffle or up through the plug and randomly blow out that area, in a completely dry tank, but I really don't know a good way to clean it.
Draining the tank and a new pickup filter "sock" may solve the problem, and of course, a clean pump and line.

OTOH Fuel pressure regulator?