You mention the radiator was flushed - which I've done by taking it out and running the hose through it.
But - what about the engine and more importantly, the heater core itself?
Not easy, but one of the "Related Questions" below specifically addresses getting the core good and clean.
Also, have you checked out the blend door / blend door actuator motor operations.
A cool running engine isn't all that bad a thing - I had to put cardboard in front of my 87 wagon when it was younger - but of course ya' gotta have heat!
Water pump defective.
If your radiator isnt leaking and car is having a over heating problem and coolant spewing out. Chances are your thermostat needs replaced. possible the radiator needs to be flushed. But more likley its thermostat
System airbound? Coolant level? Temperature control cable?
cooling system needs flushed out and a new thermostat installed, also if the radiator is still original it probably needs replaced or rodded out.
Could be your electric fan on the radiator is shot, or the relay is bad.
you must be a wierdo.stupid.
The defrost is cooing your coolant by circulating the fluids through the heater core. Check the water pump to insure that its pumping and check the radiator for blockages in the fins. Also check for an air bubble that can happen when a thermostat is replaced.
Have flushed complete cooling system replaced water pump radiator thermostat clutch fan added 2gal straight anti freeze then filled to capacity with water and truck is still over heating. What else could possibly be wrong
you might want to check for a blown head gasket or a cracked head or block. espically if it loosing water out of the radiator with out leaking on the ground.
Test gauge function with a mechanical gauge to get actual engine temp.
Low coolant? System air bound? Hoses collapsing under pressure? Water pump not circulating coolant?
I had similar problem with a mercury tracer I owned I had two problems with mine, One the thermostat was faulty(was old and rusted out) and the temperature sending unit wasn't working right ( Fan did not come on at needed temperature 205 degrees) so I had to replace them both. Hope this helps