I've asked a number of knowledgeable mechanics and we're all stumped. Figure this one out and you win the cookie. :)
Symptoms: At normal driving temperature, the temp gauge fluctuates between "Normal" and "Red Zone." When the temp gauge goes UP, the heater blows COLD. Rev up the engine a bit and the temp gauge goes back down to "normal" and the heater blows HOT.
We've replaced: The Radiator, The Thermostat, The Water Pump, We've flushed and back-flushed the block, the Radiator and the heater Core. The Coolant is new 50/50 mix and we've properly bled the air out of the system.