the condition of the coolant...may have mucked the operation of the
thermostat...change the coolant...have the heater core flushed of any
debris...insure good flow though the cooling system loop..even run without
the thermostat in place (not reaching proper combustion temperatures) to
see if it still overheats.....if the coolant is rusty, it is because someone
"panic" refilled with tap water and did not drain and fill with the proper
50/50 glycol/de-ionized water....aluminum heads will disintegrate with only
plain water....you want as fresh a coolant in there as possible...if you do not
have good coolant flow could be the waterpump has disintegrated as well~