Loosing fuel pressure

Maybe trying blowing some air backward through the fuel line to clear out the sock filter in the fuel tank. If it runs normal after that, for a while at least, then you know it's at least related to a restriction somewhere BEFORE the fuel pump. It's possible you also got a defective new fuel pump. I just replaced my mech pump with a Carter Street/Strip mech pump for the same reasons as you're describing. Holding steady at 6psi now.