Air in fuel rail

Our 95 Windstar 3.8L has intermittent drops in rpm and shuts down
completely at times. The fuel pressure is always good. We have a new
fuel pump and fuel pressure regulator. The scope shows the injectors and plugs are firing normal. The only thing really puzzling is the air in the fuel
rail. With a gauge and bleeder attached we always have air with
the gas. We're guessing this air in the line is causing it to cut out. The tank is halfway full so the fuel pump should not be sucking air. What are we missing here???

I would guess that when the engine loads up, (i.e., uses more fuel) the fuel pumping system is not keeping up. If the pressure drops, especially if the fuel is warm, vapor is formed ... and once formed, not so easy to recompress into liquid.

So, I guess, you have low fuel pressure at times ... maybe because of a failing pump, dirty filter or pump strainer. ... or erratic electrics, relays or what.