had an emission test done and failed with a P0420 error code. help plez on 2001 Cadillac DeVille DTS

I am fairly knowledgeable about automobiles. anyway,
wondering how you would stack up the most likely causes of this error code that I think comes and goes and seems to occur when I put the pedal to the metal. The error is easily resettable by me and stays away for weeks or until I punch my caddy.

Asked by for the 2001 Cadillac DeVille DTS