Surging at idle

[carstuff]

Hello and thanks for helping,
1994 Chevrolet 1500 Silverado, 2wd, 5.7l, V8, 114k miles, AT.

There is a surge at idle when the truck is warm (the warmer it gets the worse it is), it does not matter if it's in gear or in park.

We just finished putting a new intake manifold on it, set the timing to 8deg advanced (because we are at 7000 feet), put new a/c delco spark plugs, new dist cap and rotor, cleaned the injectors and the throttle body. We just changed the fuel filter and that didn't help. We also replace the EGR valve and that improved the whole performance of the truck.

I also ordered a set of spark plug wires, which should come in next week, but don't think it's that because the truck runs perfect the rest of the time. There is no Service Engine light on.

What can we check next? on a modern truck I would think it's idle air control valve but on this truck I have no clue.