1998 Chevy Truck Misfire (Cannot find Problem ) Help

I have a 1998 4x4 Chevy V6 (a 4.3 I think).
It misses when crusing about 1500 RPM and above, also when you accelerate. It also loses power if you floor it all the way.
This only happens when:
    - After a car wash     - If the weather is humid     - If the weather is below freezing.     - If the temperature changes fast (Morning dew)     - If powder snow go into the engine (-10 C)
Will Not Misfire if:
    - The weather is above -10c     - Summer (But lack power in passing gear)     
This problem come and goes, it will be ok in the morning when I go for a short ride, let it rest for 5 minutes or so. Go and the problem will be there for another 5 to 10 minutes then disappear.
I have code 147 and 300.
I have changed the plugs, Wires, Coil, Air filter and the Fuel Pump One year ago (it still make a high pitch noise).
Chevy dealers here, just wants my money. They cannot duplicate the problem and I'm not in there priority list and since I never got the truck there, well you know. All they say is give me more money and wait.
Any help would be appreciated..I really don't know what to do.
Thank you
ProTurbo
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If you mean codes P0147 - O2 sensor heater circuit Malfunction (Bank1 Sensor 3) and P0300 - Random/Multiple cylinder misfire detected
Check the wiring harness where it goes under the vehicle for a spot that has burnt/rubbed through and that is grounding out the 12 volt heater line to the cat. converter sensor. Sounds like the wire is shorting out and cutting power to the ignition causing random power pulses leading to misfires. Shouldn't be to hard to find. Take a dry day and put it up in the air enough so you can get under it. Go along the harness and look for the area that is damaged. Mark it and check the rest of the harness. Repair the bad area. Test vehicle.
Steve W.
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