How much vacuum is it pulling? My car would accelerate fine, roast the tires off, etc. It ran great at the higher RPM's and idled fine, but if you were driving below 1600RPM it would jerk, buck, and hesitate BAD! It turned out to be a vacuum leak.
How does the car run if you are cruising at a steady speed in the lower RPM range? Does it feel like you have to race the engine to keep it from surging? Now that mine is fixed it will drive at 35mph in 5th gear with no bucking at all (not that I drive it that slow in 5th).
It may just be a bad IAB as Jim stated, but I'd eliminate evey easy fix you could before buying any expensive parts.
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