I read this thread when i was at school 6 months ago when i was trying to fix mine doing the same thing. I couldn't find it once i got home. Anyways I fixed mine, and it seemed to be doing the samething as yours. What it was was the air bypass valve on the bottom of the intake manifold. i guess the spring had worn out inside it or something, but i replaced it with a different one, and adjusted it as best i could (took a few tries). at first I set it too high and always idled high, at stoplights it would cut the fuel because it though the truck was still slowing down. once i got it in the correct range it ran way better. I still notice that when its cold it is still a slight bit sluggish, but we are talking minuscule, if i hadn't had the problem i probably wouldn't have noticed. Just for the heck of it, I am going to swap the later model upperintake plenum with the bypass valve connected to the throttle body as I have heard it is less troublesome, that style plenum offers a slight increase in throttle response and because I already have the parts.
