When I shut the car off the tank boils over and I lose coolant

When the coolant tank is full, the car temperature seems to be fine. As soon as I shut the car off the coolant starts to boil in the coolant tank and I lose coolant. Also when idling the temperature tends to go up to a little over half way. No apparent leaks when the car is running except from the coolant after I shut it off. Radiator is about 4 years old, fan is new, water pump is about 6 months old, coolant tank replaced 4 months ago, thermostat is about 11 months old (got another thermostat since it was under warranty, replacement pending). What to do?