Losing fuel pressure on new setup

If your pump is sucking air, this is exactly the symptom.Generally it will suck air at that little short jumper hose right at the tank.This will not show up as a leak because its above the liquid line. It will show up better at idle. Once the Rs are up the pump tends to draw more fuel than air and it takes longer to show up.The higher the Rs, the longer the time delay.At next morning start-up the air in the line may have migrated back to the tank, so most of the line may have fuel in it,and the pump fills the bowls and pressure comes up. As the fuel gets idled away, the pump again brings up the air, and the pressure drops.
I put double back-to-back, 180*offset gearclamps, on that little trouble maker, and change it out every couple of years.
It may not be THAT hose. It could be any hose on the suction side, as well as the in-tank pick-up(that part above the liquid line).