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If the converter clogged it can cause this problem so it sounds like that is what you are dealing with. You can cut if off and run a straight pipe if you do not have emission requirements in your state. You can also remove the oxygen sensor before the converter to see if that helps by relieving the pressure if so replace but I'm pretty sure that is your problem. Let me know if you have questions, thanks.
I would put a straight pipe on the cat just to verify it is the problem, but it sounds like it. Let me know if you need anything else, thanks.