99 gmc z71 5.3 idle surging

[99 z71 5.3]

I have a 99 gmc sierra 5.3. The engine surges from 400-900 rpm while in PARK. It DOES NOT do it while in gear. It only surges about 70% of the time. every once in a while will idle just fine.. i notice it occurs most when i turn ac on and hit the brake or try turning the wheel. Ive replaced IAC, TPS, MAF, intake manifold gaskets, fuel pressure regulator, fuel filter, and throttlebody gasket.. truck does have a cool air intake, throttlebody spacer, and headers. Has anyone had a similar problem or have an idea of what could be causing it? Any help would be greatly appreciated.