Hey,
My 99 Grand Am does the same thing. I took it to the garage where they ran diagnostics and couldn't find anything wrong. It only does it after driving for atleast 20 minutes and while going up hills. Also while driving high speeds. When you give it more gas to keep up the speed, it hesitates, the RPM's bounce up and down and the only thing to make it stop is to give it a ton of gas. My dad, who is a mechanic, believes it to be a problem with the fuel pump itself. I haven't felt like taking it to the shop again because I am planning on getting rid of the car ASAP. I've had more problems with this car than my old 89 Mazda! Good luck finding out what is wrong with your car, just wanted to let you know that mine does the same thing...