[BR3W CITY]

Check the pressure again with the vac line disconnected from the rail and see if it changes (bad reg).

Put a multimeter on the pump when its idling, and see if there is an anomaly that occurs when the pressure drops (i.e. pump voltage drops, or pump amperage spikes).

A spike in amperage usually means its drawing waay too much, and there is undue stress on the pump. First place to look is for kinks/bends, and that the fuel filter isn't plugged up (and that its properly rated for fuel injection!). Even with a "new" or replacement tank, there can be junk floating in/built up in/leftover from fab. This stuff all gets stopped at that new fuel filter, but the filter doesn't get changed till months/year later when they get around to servicing it.

If there is a voltage drop, then your power feed to the pump (or ground) may not be substantial enough to carry the voltage, or there may be an electrical load elsewhere on the system causing it to lose priority. You can temporarily bypass ECM control and "hotwire" the pump with constant 12v from an external source to help check this if powering is suspected to be the problem. Cycling heat/radio/wipers etc to create draws and watch for changes also helps