getting any strange gurgling noises from the car? If so, could be a stuck thermostat.
Otherwise, sounds like a pressure leak. When the car warms to full operating temp, is that when you get the leaks? Might be a crack in the cooland resevoir, or even a pin-hole coolant hose leak.
Next time you get the car fully warmed up, shut it off & pop the hood. As the car sits for the first several minutes, the coolant temp will rise a bit more & pressure will build a little higher before the cooling off phase begins. That's when you'll find the leak.
