[HardcoreZ28]

Does it always happen with the throttle in the same basic position? I'd check your TPS for proper adjustment and operation. It's free to check before you start shotgunning parts into it. Probe it with a meter and check the min and max voltages (not sure what they are supposed to be for a 4th gen). Then with the key on/engine off open and close the throttle and watch that the voltage sweeps up and down consistently.