Even GM mechanics cannot figure this overheating problem. Already replaced radiator, thermostat and water pump. Gauge goes into red and drops when accelaerating. Tried to bleed air from loop. Any suggestions?

If it overheats when you are sitting still idiling, and drops into the normal range when you are driving down the road. You have a bad hydrostatic fan clutch, i.e. when you are idiling your fan is'nt pulling enough air across your radiator.

Maybe a faulty fan clutch? Assuming there isn't a major problem like a blown head gasket. Does the truck take a normal amount of time to reach operating temp? If so, let it get warm. Then shut it off and try to spin the fan blade with your hand. At operating temp, the fan should only spin about 1 blade. If it spins freely, the clutch is bad and is allowing the fan to freewheel when it should be "clutched"