I have a 1996 Pontiac Grand AM, 4 cylinder. It has been idling rough, surging from low to high rpms and back and forth for a bit. It then immediatly dies after this. The only time it won't die is when I give it a large amount of gas, up to 5,000 rpms for several seconds. When I do this it will continue running. I have already had the battery replaced. What do you think it could be?