Engine misfire

I have a V-6 2.9 Automaitc 1989 Ford Ranger. When you first get in and drive it will run smoothly without any problem. After a few minutes though it looses power. It will barely run 35 mph with the peddle pressed all the way the floor. Plus it will also start knocking real bad. What can cause this?

Not in first gear but in second and third, and possibly fourth and fifth, and when the rpms hit anything over 3000 the truck bucks and misses so badly it almost bounces and today coughed out some black smoke. It runs okay up till 3000. Replaced plugs and wires and cap/rotor and it seemed to help a little but not enough. Truck engine also never warms up, its about freezing when I go to work and it stays cool for the fifteen minute drive.

Your vaccum timing advance might be on the fritz. That would be the cheapest thing to fix so hope its that. Check your vaccum pressure. The 6cyl has a 2 barrel carb right? Maybe one of the jets is blocked and your engine might not be getting the right amount of air/fuel mix. It could also be the little resevoir and float acting up and not letting enough fuel get in.