1999 Isuzu Trooper Hesitates on start up

When I try to start the car, it turns over and sparks, but does not start. It only starts after I pump the gas a couple times. If I do not rev the vehicle instantly to above 1k rpm then it dies. Also occasionally, at stop lights and switching gears the rpm will drop to lower than 500 and the vehicle will sputter. Usually it will immediately rev faster (up to a steady 750-1000). Though, it will occasionally die at these times.
The problem has occurred in the past, but stopped. I recently replaced the PCV valve, the fuel filter and a disintegrated (overflow?) Fuel line. I can't figure out where the problem is and info would be greatly appreciated.