Well, hmmmmmm.
How about the idle mixture, set for most vacuum. For that matter, what does the gauge read when this happens.
Does this go away when the truck warms up, after 20 minutes or so. Do you have some kind of manifold heat?
Have you set the timing using a road test, as per the shop manual? Your timing may be to retarded. What with todays modern gas and all you might just find that you can move the timing up a little more.
