Missing under load

Typically the fuel pressure varies with manifold pressure, that keeps the differential pressure at the injector constant, so the fuel delivery is always proportional to time of injector pulse width.

Is the pressure reference hose connected and leak free to the fuel pressure regulator? Also is the fuel return line working correctly? The fuel pressure should vary up to about 15 psi, depending on pressure reference for a normally aspirated engine. It should be lowest at idle, and increase as vacuum drops.

Fuel trims in the ECU get messed, if things are wrong. It might take a reset, after you fix something.

Please ignore my comments, they to not apply, and are incorrect to PO fuel system