hard jerk at low and higher speeds on 2002 Saturn SC2

When going down the road, if you stay steady on the pedal the car will drive smooth except for on the interstate at about 70 miles an hour then it will jerk real hard and then smooth out. If you give it a decent amount of throttle at low speeds, you get the same response. What is causing this?

I'm thinking ignition system problems. Check the plugs, and the sparkplug boots for integrity.
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minw\e does the same?