1998 plymouth voyager running hot. water in resovier boiling and steaming. put kw in still running hot.?

Everytime I go places I have to check the water in the radiator and resevoir to make sure it is not low so I could drive safelly. My uncle says its the computer but I think it is not. tried everything. replaced the thermostat its ok. water pump ok. not sure about the radiator except it was drained and applied more coolant. still the same way please help.