1997 Mitsubishi Eclipse Coolant boiling with out overheatin

I have a 97 mits eclipse rs with the 420a engine, the coolant keeps boiling through the overflow container, i have an aftermarket metal radiator, i had a check engine light with an evap error, i put block sealant in the radiator that was safe with the coolant and the check engine light has gone away. Yet I still am boiling the coolant the gauge does not read hot and the computer sensor for the temp has been replaced about a year ago both radiator fans work. I have no idea what else to do other than check the theromostat. Penny for your thoughts on why the coolant keeps boiling over.