94 Astro van 40 to 55 psi fuel pressure will not start unless gas is added toair duct will run till turned off

have changed fuel pump,filter,relay,dis.cap,rotor,plugs,co… sen.iac valve,fuel meter body,poppet nozzals,fuel pressure regulator,(spider unit)intake manifold tuning valve,powertrain control module,I have no leaks,35 to 45 psi.oil pressure.I can add gas into the air duct to get it to start and it will run for sometime(15to20mins.)and I'll cut it off,if I start it back right then it will start right up,but if I let it set for more than 20 to 30 secs.,it want start back unless I add gas into the air duct again