Do you have the cap for a non-recovery cooling system (a cap for a recovery system is designed to open when hot!) and does it "overheat" if you leave the cap at the first click?

Boilover does not mean overheating.

If you are experiencing boilover, look at the cap again. Once the engine cools, remove the cap and the coolant should be about 3" under the filler neck. If you fill it to the top it will burp out the extra to save the radiator once it is fully hot.