2001 Hyundai Santa Fe code P0421

I've been having trouble with code P0421... I replaced the catalytic converter and it went away. Two days later the check engine light came back on so I went back to the mechanic and he cleared the code... then 3 days later the check engine light came back on again. I don't know whats going on... could it be that the oxygen sensors are faulty? Also when I come to a stop at a red light somewhere... sometimes the car feels like it jumps... it doesn't move forward but it just feels like some sort of bump. It runs fine but the only problem that comes up is code P0421... anyone have any suggestions? Thanks.

P0421: Warm up catalyst efficiency below threshold. This code means that the computer is detecting the catalyst not warming up to full function quickly enough based on information from the post cat O2 sensor. Was a Hyundai catalyst installed, or was it aftermarket? It sounds like you may have recieved a bad one. If it was aftermarket, I would get them to refund your money and put a Hyundai part on it.

This is the one that was installed according to the parts number...
http://ourdealsrock.net/256-99004HM.aspx

That's not going to work. You need to try to get your money back and get a OE converter or a better quality one.