98 Cavalier Z24 2.4 DOHC Cat conv code

When going for insp. Stkr the computer returned PO420 for a bad cat but the converter had already been checked and I was told the computer was bad. The inspection tech said that since computer only gave one code that I may be able to just clean a tube running between the sensors and the cat itself. This, he said, can get clogged or blocked when a car has been sitting as had been the case. Has any one experienced this and can they direct me to the tube or its cleaning? Also the car weakens, idles bad and stalls after being driven 20-30 mi. During traffic in hot weather then starts after cooling for10-15 min. But still labors and sound like a quiet diesel until hours of cooling. I havn't been able to catch the radiator fan running at all. It quits, starts in neu, repeats a couple of times then must be parked. From time to time it also indicates Low TRAC on the intru. Panel. Could this all be connected and remedied as the insp. Tech indicated? I'm most curious about the alleged tube or pipe that is some where around the O2 sensors.
I appreciate your expert forum.