[JP Noonan]

Try pinching off the return hose after the fuel pressure regulator when you notice the pressure drop. If it increases pressure I'd say the regulator is shot, if it dosen't help then the pump may be dying. Watch the gauge, you don't want to pinch it off so much that fuel pressure spikes past 35+ psi. Above 35 or so you may repture a fuel line or blow off a hose clamp.

The problem with using a gauge not made for gas is the internal parts and seals may desinagrate from the fuel and then rupture spraying gas all over. Where safty goggles.