Why don't you replace the cat? Seems to be the reason the car runs like crap, the rear bank requires a working cat to actually fire correctly, this is a known fact. That is why all the smart guys on here suggested you replace it, cause clearly that's the first thing everyone would do. Every time i have a problem i just replace the cat, a/c wasn't working, i replaced the cat, bam, cold air conditioning again. I had a flat tire the other day, i replaced the cat, bam, no more flat tire.