2002 Chevy Cavalier Engine Sputtering on Freeway

My Cavalier has been running excellent since I've had it until about 2 weeks ago. I was on the freeway and driving at about 70-75 mph and the engine sputtered. I didn't think anything of it and a couple of days later it did it again a few times in a row, and when looking at the RPMs they will drop as much as 500. I took it to a dealer because the check engine light did not come on. They recommended that I change my plugs and wires and this did not help the situation. When the car is at slower speeds, it is fine and has plenty of acceleration. The problem only happens on the freeway after I have been driving for about 30 minutes.