WRXSTiSpec-D

Hello everyone. New to the Forum and Subaru ownership. Please be gentle. I have a 2007 WRX STi Spec D. After a short time the car started to throw up a CEL and an error code of P0420. The car is, as far as I know, totally standard. It passes the MOT emission test no problem, so my first thought was that it was the 02 sensors. They have both been replaced with new ones, but the CEL still comes on randomly, not all the time. I'm thinking if it passes the MOT emission test, the CAT must be OK? What would the next course of action be? I do most of my own spannering, but I don't want to go down the road of randomly replacing parts, most of all the CAT. I have checked the inlet and exhaust for obvious leaks. I have done a search of the forum, but can't find anything too relevant. A google search suggests the EGR valve maybe dirty or faulty. Any thoughts please?