Code P0420?

I am getting a check engine code of p0420 from my OBDII meter. I know this suggests a problem with the cat. To eliminate other possibilities I have changed both 02 sensors, cleaned the MAF, replaced spark plugs, and it is not yet in need of an oil change, I even hooked up a smoke machine to check for leaks I found one small leak and repaired it as well. After each repair I erased and reset the code meter. But before the car completes a full drive cycle the light comes on again! I am skeptical about the problem being the cat because I am not experiencing any of the other normal symptoms related to a bad cat listed. Ie. Sulfur smell, exhaust smoke, poor gas milage, low power, trouble starting, etc. Symptoms I am having which are probably unrelated are occasionally idling rough while in gear and at a stop, (shifting into neutral or park stops the rough idle) and occasionally having a high-speed vibration, if the light were not coming on I would not suspect any issues. I am trying to avoid replacing the cat but have run out of other possible things to remedy the check engine light and I am past due for my smog check to register the car! Please help!