Plugged filters, tank strainer, tired lift pump, air leak, Fuel line obstructed. My guess is the fuel filter as the car runs for awhile until the junk in the primary or secondary filter gets stirred up and totally obstructs. The filter is obviously always partially obstructed.

I think your injectors are not the cause of your symptoms at all. They might not be perfect with age but cannot give you the present problem themselves.

That why even with your 14 pound output electric fuel pump you had such poor readings in the injection pump. If your gauge is not a fluid dampened type gauge you are on your own as the non dampened ones are almost useless for troubleshooting in that application.

Read the pressure close to the electric pump. Than the injection pump base. If there is a large difference I think the secondary filter you never changed or mentioned you did since the problem started is a possible culprit.

You have been mixing fuels in that car as well. This would get a lot of junk in motion. A secondary filter required every two weeks or so would not be unusual.