[Super73]

Is your regulator before or after the fuel rails?

I have first hand experience of fuel pressure falling off a cliff when it is before the rails (same regulator but different pump).

What happens is when the regulator is before the rails, it has to try and switch fuel from the return to the rails quicky or you get a fuel pressure drop as the motor takes a big gulp of fuel.

Putting the regulator after the rails it only has to return what is not being used.