my car is doing the same thing, check your water pump.
my water pump is messed up and leaking/letter air in. Only a very little but when its sitting and cooling it sucks air in rather then letting the system suck it from the overflow.
I did the 'bubble test' and nothing (other then the air thats in the system :P)
Top off your coolant daily as the air can build up enough to stop coolant flow.
You may not see it at first, i didn't. But after scratching my head for nearly a week i saw the slow leak comming form the water pump