89 2.9L kicks & bucks at cruise

[patrick260z]

I've got a 89 2.9L V6 Ranger. At cold seems to do city drive ok, but misses under acceleration to highway speed (heavy throttle). Once up to temperature, shutters and bucks while cruising. If I leave my foot in the same position the truck goes from 70 down to 60 before coming to life again, and accelerating to abt 65 before starting to spurt and sputter. If at cruise and the truck starts sputtering and I give it more gas, it will all of a sudden roar to life and accelerate. In fact, if I drive the motor above 4k, no shuttering or anything. Once warmed up and I drive around the city, the vehicle is always bucking... unless I keep it in too high a gear and grunt it around. So it seems like the vehicle, when warm, runs smooth only under load.
I have replaced:
fuel filter
plugs and wires
fuel regulator
MAP sensor
air filter
and I have done a couple of seafoams too.

Does anyone have any idea whats goin' on??
Thanks

 

[Gary101386]

clean your IAC Idle Air Control. had the saem issue with my mustang and it fixed that.