2000 Other Oldsmobile Models Car "bucks" or pulls

While driving my van will "buck" or "jerk". It does this while simply driving or when you have to let off the gas & then accelerate again. It was hooked up to a computer & it showed cylinder 2 was misfiring quite often, replaced the coil & wire, this fixed it for about a week. Then is started getting bad again, sometimes jerking hard enough to feel as though the transmission was going to fall out. Yesterday I got a tune up, replaced the plugs & wires, air filter, fuel filter, a PVC "something" & flushed the transmission. This did make it better but I had an episode when going around a curve that had a "dip" in it this morning. It wasn't as hard of a buck but it was noticable. What could this be? The only other code that shows up is the Catalytic Converter, I just had one put on (it is after market & was told that is why the code keeps popping up) and I have replaced the back oxygen sensor. Help