What is the codes you are getting and what have you done so far?
Ok, Take out LH bank #1 02 sensor and clear the codes. Leave the sensor out start the car and take it arourd the block. (It will be very loud) If the misfire codes doesn't come back on. It is your cat that are cloged up. you will get a code for the 02 sensor because you have it out. I had the same thing and it drove crazy it took me 4months to figure it out. I cut the LH side cat out and put a new one in and the car runs great. Let me know how you make out.