1998 Chevy Camaro Cylinder Misfire

I have a 1998 V6 Chevy Camaro, which has had some problems lately. Recently, when I have been starting from a complete stop, it seems as if it was having trouble getting going. It seems to jerk back-and-forth, and once it gets going, its fine, it also doesn't have much power to speed up. It also "rattles" when it is idleing. I have recently gone to Auto Zone, where they offer a free report/test, and bought a part to replace one that wasn't emitting a spark ("Cylinder Misfire Detected - Random Cylinders" -- We replaced the ignition coil for 2 + 5 since it was not emitting a spark once the engine was turned on). Thinking that was the problem, we tested it again with the test/report device, and it passed. As I was leaving the auto parts store, the check engine light went on, and flashed, thus, telling me the problem was not fixed. Also, I can hear a wierd sound while the car is running, in the rear of the car, and I was thinking that it would be the catalytic converter. Help!