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P0430 code in 2000 GMC Sierra - Grrrr

2000 GMC Sierra 1500 4X4 w/ the 5.3L V8, 127k miles. Keep getting a P0430 code with the SES light. Have replaced both bank 2 O2 sensors and plug wires, and had fuel system/ injectors cleaned which eliminated a slight miss in the engine. Pickup runs great but I have this annoying light that keeps coming on. Strangely, and this may be relevant, I can clear it and hit the highway and while driving at, say, 70-80 mph, it does fine. The light only comes on after I pull off the highway and allow the engine to go back to idle speed. If I clear the light and just drive around town, never going > 30-40, it doesn't come on. Also, and I don't know if there's a correlation, it only has come on when temps have been
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