Code 51- What to do...

[LeeNeon]

Hello,
My car has 82000 miles on it and is giving off a code 51 indicating that my fuel system is lean. Immideatly after the PCM has been reset (until the time my CEL comes on) the car runs well, seemingly normally, throttle is smooth and it pulls right to redline (making me think it's not fuel delivery). When the light does come on (I think the PCM goes into failsafe), the car noticably lurches when you come off the gas, however, it pulls fairly well under full throttle (not quite as strong as when the light is off). Shortly before my CEL first came on I noticed a rotten egg smell coming from my exhaust, I'd assume this is my catalyst failing, two days later the CEL comes on. I would have replaced my cat first but I can't see how the catalyst could be causing my engine to think its running lean. I took my car in and the mechanic changed my fuel filter, and said my O2 sensors were functioning normally, I then changed my plugs, and replaced my MAP sensor with the extra one I had lying around from my other used IM. I reset the computer and 40 miles later the code came back. The code has persisted through 2 bottles of injector cleaner, and 3 tanks of gas (all of which were different brands and levels of octane, and the PCM was reset).
My question is what should I do? I am planning to replace my cat but I was wondering if there are any more likely things causing the code (as replacement is rather expensive).
Can someone try to explain how a broken catalyst could be causing my car to think it's running lean.
Can anyone reccomend a good replacement cat, prefferably high flow and OEM functioning.
Thank you in advance for your advice,
Lee McCay