I have twin 95 502 mag efi motors. I go from roughly 32 psi and when speeding up I slowly go loose pressure till under 18 psi in 1 motor. I swapped the vst and mechanical pump and still does it. The sea water pump was replaced early this year. I also replaced the fuel line to tank and the anti syphen valve. Not sure what to do next? And recommendations? Thankyou

Complete VST with new electric fuel pump ?

I swapped vst from motor to motor to try and diagnose but still did it

start at the fuel pick up and start looking.I had one,well 2,1 was in my car,,the antiphon valve got all clogged up and restricted fuel flow.also look at all the fuel lines for weak/collapsing.
or try a new fuel filter.

Sounds like you need a fuel pressure regulator.