Jeep Grand Cherokee has hesitation and misfires when starting out

I have 2 Jeep Grand Cherokees, a 93 and a 98, with 6 cylinder engines. They both have the same problem. When starting out they hesitate and jerk. If you keep pressure on the gas pedal and slowly push it down it will eventually pick up and run. The cap and rotor, the injectors, plugs and wires, fuel pump and filter have all been replaced. Still the same problem. Any ideas would definitly be appreciated! Thanks very much.