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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 <32-degrees. Then again, it hasn't been warm long enough this month to see if it'll do the same thing when it's nice outside. This problem began about a month and a half ago when the fuel line froze during that really bad cold snap. When we finally got it to turn over and run, I had to replace the number 2 coil pack, replaced the wires at that time, and it's been throwing the P0430 code light ever since. Like I said, the truck runs great, no loss of power, no misfires, good idle and the mileage is back to normal since replacing both O2 sensors. SHould I look at the simple stuff - new fuel filter, new plugs? Or, what? Not a mechanic, just a po' boy trying to keep his ride running.
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