Hi Im Larry Crow,
The 4.3 engine in this vehicle has a high fuel pump pressure requirement of about 60-65 psi. I suspect that its to low and or the fuel filter is restricted. This is a common problem and I think you will find your self replacing the fuel pump and filter. Run a fuel pressure gauge on here and if its below 60 psi replace the pump and always replace the filter when replacing the pump or you will not get a warranty on the pump.
The lean condition of low fuel pressure will cause the random misfire code but the o2 sensor code will be a hard fault in the sensor and will need to be replaced also. The O2 sensor will not cause your power problem, but it will affect your gas mileage. This should get you down the road to a solve for your symptom. If you need more help please ask me and I will be glad to help you. Thanks again for using justanswer.com
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I know its a pain to drop. You have increased your fuel pressure though and may not have to do a pump. keep your fingers crossed and let me know how it works for you.
FYI It always seems that when we have to do a fuel pump the tank is always full.
One more note , the manufactures that make these pumps claim that they will last a lot longer if you keep the fuel filter changed every 10,000 miles and always keep the tank above 1/4 tank. Good luck and let me know how it goes.