99 Bucking when hot, constant throttle or under load

I have a 45k mile NB1 that has been bucking when it gets warm. Cruising it’ll bounce around a tiny bit and then when it gets past 3k rpm it starts to hesitate and buck. This doesn’t happen until the engine warms up, and it doesn’t happen every time I drive it.

I have a po421 code, I have changed plugs and wires, and my next step will be the fuel pump. I have heard it may be coil pack or injector related, or possibly CAT or bad 02 sensor.

Any advice or things to look for, order I should replace parts etc would be appreciated