I have the po420 code on my van. It is a 96 astro 2wd, 4.3 vortec. Also has a little bit of a miss when going up an incline on the highway with the cruise on, under light throttle. Doesn't do it at idle or if u power brake it. I was having some cold start problems and just replaced the fuel pump, filter, and pulsator, that fixed the cold start problem cleared the code and it came back. Any ideas? replaced the plugs, wires, cap and rotor about 30k ago. The heater will burn u out, but the temp guage hardly gets over 150 replaced the thermostat about 1k ago. I need lots of help here guys lol. Thanks in advance