1993 Jeep Cherokee Fuel Pressure

I have replaced fuel pump filter checked vac lines changed idle air valve throttle sensor. what i have got is when the igniton is turned on the pump puts about 30 psi into fuel rail but when u start the jeep the pressure goes down about 2 psi a second tell it final dies and there is no pressure in fuel system then u turn ignition on and pressure is back tell started. what can be cuasing this?? there is also alot of sucking going on in the intake and backfireing. it is sucking in alot more air then it used to.