Without a scan tool to see what the computer is seeing all I can offer at this point is to run some Exxon/Mobil 93, two or three tanks and see if that clears it up. If not, it's probably time for new cats.
Stay away from the cheap ones, they don't last. I put a Magnaflow Y-pipe with high performance cats on mine and sold the old ones to the scrap yard to help offset the cost. Factory Y-pipe with cats is north of $900, and
@GotExhaust.com made me a good deal. It was a pain for this old man to install it, I had a mobile mechanic come out to the house and finish.