if it was the headgasket, youd have worse symptoms then it just getting a little hot. when you say it gets a lil hotter, how much are you talking about when it is fullywarmed? Like the other guys said, the problem is either in the coolant or the thermostat, good idea to do both at once. The thing about the thermostats is, while a car may have low miles, it is still 10 years old so sometimes in the thermostats the spring gets stuck open (which causes the car to run cold) or the opposite.