1999 Lumina Hesitating/lurching/bucking

My wife is complaining her car is Hesitating/lurching/bucking when she gets on the highway at about 45-50mph. If she accelerates she can seem to get past the problem. We had a similar problem last year but the engine light came on and it was diagnosed as ignition problem. I replaced plugs and wires and it's been fine for 8k miles. I used AC plugs and don't suspect this to be the problem again. The engine is a 3.1 automatic transmission.
Any help would be appreciated