Overheating

The car is running good but when we stop the reservoir for the antifreeze is boiling and after it cools down it needs to add antifreeze. The thermostat was replace three weeks a go and the problem stop until yesterday when it did it again. The fans work and we have done a test to check for head gasket and it was negative.
And id does not do it all the time. I can drive it all day and it will be fine and then the next day it will do it again.