Could be the radiator not opening when it should. Could also be the water pump. I had a very similar problem last summer. First got a leak at the expansion tank. Got that fixed and then I got a leak at the bottom radiator hose. When I brought it back to the garage I told them to check the thermostat and water pump. They did not, just got the hose and coolant replaced. 2 days later the bottom hose exploded. It ended with having to tow the car and get entire cooling system (Radiator, Water pump, thermostat, etc...) replaced. Fortunately my extended guaranty covered everything.