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. Any suggestions will help.

 

I have similar problem with water loss on an 02 Millenia. I also changed the thermostat ....I'm very interested in any replies that you get,

 

This might be a long shot, but check your oil for signs of water drops on the dip stick. If there are water drops, you might have a blown head gasket.