Does my 2000 Honda Accord V6 have a bad Cat Converter or a bad Oxygen Sensor?

I recently bought a 2000 Honda Accord V6 4 months ago. It has been driving beautifully. A few weeks ago I was driving the car and it began to shake really badly and I was able to get home. The next day I got in the car and started to drive and for a few miles the car would seem to lose acceleration. For example I would push on the gas and the car would go, and then the RPMs would drop even though my foot was still on the gas and then suddenly start back driving. After a few miles it would start driving normally. The check engine light came on and it said that the upstream oxygen sensor was bad (bank 1, sensor 1). We had the oxygen sensor replaced and the car drove fine for a while. Now it is back doing the same thing and the check engine light came back on and is saying that the same oxygen sensor is bad, and we just replaced it. What could it be? Is it the cat converter? Also, some other things I am noticing is bad gas mileage, a burning smell while driving. And some other strong smell coming from the exhaust.

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