Why does the car buck and shudder?

The car works fine other than this one problem. It shifts beautifully and runs well until I get to a cruising speed on the highway. After letting up on the accelerator and coasting for awhile, when I try to re-accelerate the car bucks and shudders until I let off or punch it into passing gear, and then it is fine again until I cruise and re-accelerate again. It occurs most often in the 80 to 100 km/hr range. I am not quite sure if it has to do with the transmission and or controls, or the engine gas delivery or controls. I am aware that having codes checked would be a good place to start, but I was hoping to get an educated opinion first. The check engine light does come on but does not always stay on. What do you think, any ideas? Thanks!