1999 Nissan Pathfinder Exhaust

A few months ago my check engine light came on. I reset it ( by unplugging the battery) and about 700 miles later it re-lite. So I had the code read and I was told it was a code-420 (or something to that effect) and the technician reset the code telling me that my catalytic converter is preforming at less then optimal, but save for lower fuel efficiency it isn't a concern. I reset the light 2 or 3 more times (it comes on every 150-200 miles). Then all of a sudden the check engine light is no longer coming on. It has been over 1500 miles since the last time I reset the code. Is it possible the cat was only clogged and it has since become unclogged? I think I am getting better (although not as good as 100,000 miles ago) gas mileage. Is it possible for a cat to become so clogged that it registers on the system then become unclogged? Should I still look into having the cat replaced?