1996 Mazda 626 Coolant loss

I have a 1996 626 with a 2.5 V6. Car runs at normal temp when driving and I hear the cooling fan come on , when I turn car off coolant system likes to bubble and backup to the reservoir and leak out the overflow hole and then I refill system. I have installed a new thermostat, cooling fan works, water pump seems to be circulating coolant, caps seem to be holding pressure. Its when the coolant has gone and the car overheats that is concerning me because it is usually without warning unless I'm lucky enuff to see the puddle under the drivers fender. I have watched the coolant rise right out of the reservoir when I turn the car off, it's a dance that is getting tiresome. And the car has heat also. Any suggestions will help.