'97 code P0420 (catalyst efficiency)

check engine light came on, had Advance Auto parts run a free diagnostic. The code P0420 came up which according to the scanner and manual is catalytic efficiency. I've noticed recently that the when the engine is cold, the idle fluctuates between 700 and 900 RPMs (in a steady rhythmic pattern) only when in gear. Headlights seem to dim and brighten along with idle fluctuation. Any ideas on how to trouble shoot the diagnostic result and does the idle symptom seem realative to the code?