ideally the water should never boil, thats the whole theory behind having a thermostat and a cooling fan.

The only time you should hear/see bubbling in the header tank (IF at all) is just before the fan cuts in and it should clear almost instantly.

Too much water in the tank shouldn't really make any odd's it'll create a little more pressure in the tank but shouldn't promote boiling of any kind.

As ive said above aswell, do your heaters blow hot?