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Definately sounds like the O2 sensor needs replacing. Did a Service Engine Soon light come on?
You can try to swap it with the post cat sensor to see if things improve until you can get a new one. Hopefully the new data log will show the same one not reading accurately.
FWIW, my 4.3L was acting like you are describing and once I swapped position of the sensors, the truck started running right again. The SES light was still on, but the code followed the bad sensor. Now I just gotta break down a get another one...
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