the OAR value should not drop like that, could it be bad gas? maybe? But I assume you have been fueling more than once recently so I think the likelihood is less for bad fuel. To much Ethanol is a possibility, but you would have to run north of E30 to see it, and as soon as you fueled up again with 92 it would resolve.
With the comment from SHOdded that you may not have a true flexfuel sensor for that year were back to possible O2 sensors.. your cats seemed fine as compared to your fathers. O2's they play a large role in determining fuel 'type'. are you 100% certain the dealer really replaced the o2's and did they do the fronts AND back ?