Toyota Tacoma check engine light

I have had ongoing problems with my check engine light coming on. Code PO420. I have replaced front fuel sensor, front CAT, rear sensor and CAT, fuel cap and fuel filter.The truck has 106K miles and all maintanence is current. Runs great but the check engine light randomly comes on. Now sometimes it goes off by itself without having to be reset. Going on now for 6 mo.Anyone else suffered something similar?

Model-Year, Please. The Car Talk Question Form Fails To Ask For This Important Information.

Also, if more than one engine was available, which engine do you have ?

Thanks,
CSA

It is a 2000 v/6 and as far as I know there was only one V/6 available for this model year. I do know it has california emissions.Automatic, extended cab 4WD.