Have had some misfires stored in the ECU recently. Cars noticeably having issues on occasion, feels like it loses power going up slight inclines, rpms a little jumpy for first minute of it being turned on. Has new spark plugs, and boots. 02 sensors about a year old. New Thermostat and Housing. Yesterday it's CEL came on throwing a P0420 for the catalytic converter. Are there othwr things I should be looking at that could be causing that or am I looking at replacing the Cat? Expensive fix here in Los Angeles since we have to have those more expensive ones.