My '98 Suburban, 5.7L, 4sp auto, 4wd and 176,000 miles is giving the subject code indicating " Catalyst Efficiency Below Normal (Bank 1)". It started following an episode of misfiring that was identified as code 0300 " Random/Multiple engine misfire detected". I chose to drive a couple of hundred miles with an misfiring engine rather than walk a couple of hundred miles. After replacing the distributer cap and clearing the code, the engine seemed to run normally. After a short time, the 0420 appeared. If I clear the code it will return after the driving less that 10 miles. Any idea what my problem might be?