Power loss when warm

Hi I have a 1993 Chevy 1500 2x4 with 213,000 km and the 4.3 tbi v6. It has an automatic trans. What I am wondering is what could be causing my low power issue when the engine heats up. I recently did the head gaskets on it and when I did those I changed all the top end gaskets also. When the head gaskets went the coolant burning made the oxygen sensor read negative voltages so I replaced it. I also replaced the cap, rotor, plugs and wires. My problem is that when I first start the truck and the engine is cold it has tons of power and has no problem driving, but after about 5 minutes of driving all power is lost and I have to floor the acceleraotr to get up hills and it even struggles on level surfaces. I have scanned it numberous times with both the OTC genesis and a snap on scanner and no codes are shown, o2 voltage is perfect, fuel trim is 128 and so forth. All sensors have been chacked and are within range. Can you give me any toher ideas of what it could be, like tranny maybe? Thanks alot, jamie