Loss of power on a 95 Chevy 2500 Pickup

I have a 1995 Chevy Silverado 4X4 Extended Cab Pickup with a 5.7L V8. I have owned this truck from the beginning and have had no engine problems until recently. I started loosing power when it warmed up basically just disappearing when the throttle is held steady. I have replace the fuel pump, Fuel Filter, Map sensor, O2 sensor, Throttle positioning sensor, the Distributor cap and rotor, wires the module in the distributor, replaced all the gaskets in the throttle body as well as the idle solenoid but to no avail. Originally when this started I could stomp on the gas and it would pick right up but now I have to do it several time and noticed yesterday that when it did not go that the RPM's ran up and down about 2000 but my speed did not change. A friend of mine now thinks its the transmission but again when its cold it usually runs like a top. any help would be appreciated. Thanks