It is too much heat entering the water, either your cooling system is not as it should be, or you have headgasket going/gone.
I would look at reverse flushing the rad as a minimum, then check the top bottom hose to see if they are tha same temp and nice and hot.
I would also for the money install a new thermostat.
With this much cooling there should be no reason at all why it would not keep it's cool.
When you say overheating is it pushing water out of the expansion tank, how do you know it is oveheating ?
tabetha