What Part is Left to Change?

Hi folks; thanks for the help you give.

I have a 1995 XJS - 4.0L AJ16 - 90,000 miles.

MY muffler guy says one of my cats is beginning to show early signs of blockage. I have changed the plugs, fuel filter, and had the throttle body and fuel system cleaned. I feel a random and inpersistant rough idle and my mileage is not what others get with this car. I get codes P0420 and P0430. Do I have a bad coil (there are 6 coils), a bad O2 sensor, a bad/dirty MAF, or bad injector? At running speeds, I notice no roughness or hesitation. Thanks for the advice. PS, any way to home test the items?