I have a 94 STS 4.6 Northstar. When accelerating from a stop, one never knows if it is going to go or not. When it doesn't go, it begins to knock from the accelerator and barely runs. After pulling to the side of the road the knocking will go away after a minute or so. Then I can accelerate slowly. If I give it a little too much gas it will act as if it is stalling again. The engine will idle fine but step on the throttle while in neutral and the engine will hesitate. I replaced the fuel filter, and O2 sensors. Please help me trouble shoot this problem.