Engine Cooling problem
2003 Pontiac Aztek 6 cyl Front Wheel Drive Automatic 141000 miles

Within 5 minutes of driving the vehicle the warning light comes on with high coolant temp, then would drop down a little, then go back up. Pull over both fans are running upper hose is hot, but top of the engine is cold. Replaced coolant and thermostat, bled the lines. Sometimes there is heat and other times just blows cold air. Can sometimes get the temp to drop by putting the vehicle in neutral and revving the engine.

Any ideas what the problem may be

Thanks