Lurch and uncle Fester

96 Pont Gran Prix - Lurching Hello. I really know nothing about cars, so I hope someone will be able to help me. I have a 96 Grand Prix, and it runs fine until I get up to around 50 mph, when it starts sputtering and lurching. it continues to do it, and I get frustrated, and press down on the gas more, and when I do this, it runs great again, until I slow back down, and try to stay at 50--which is the main speed around here in nowhere land. Any suggestions on what I should be looking to fix? Other than that problem the car starts quickly, and runs very smooth.