That sounds similar to what my wife'* Grand Prix was doing a couple weeks ago, If I opened up the throttle it would make a popping noise from underneath the car, and hesitate and not accellerate. It turned out to be a plugged catalytic converter. Apparently federal law requires the manufacturer to warrant it for 80,000 miles or 8 years, and in California it might even be longer, like 100,000 miles and 10 years.
This may or may not be your problem, but you can see if you can repro it by going WOT on the highway.
97 SE, teflon wiper blades, Dunlop Sport A-2 tires, Leather steering wheel cover, Pioneer DEH-P480MP mp3 CDRW head unit, Pioneer 12-disc changer, iPod hookup, Boston Acoustic 3 way 6x9'*, Boston acoustic separates in door pods, Alpine 4 channel amp in trunk. Magnaflow dual outlet muffler and Ractive stainless tips. Autozone rice pipe and K&N 4750 intake kit. Clear corner lights. Energy Suspension end links.