2.4L. I've replaced the thermostat. The oil looks good (ie, not milky). I made sure it was full of coolant. When the car is cold I can start it up and let it idle for 30 minutes and the temp stays fine, the radiator fan is running. If I take it out on the highway it will run fine and the temp is normal. But when I then (after the highway run) either let it idle or drive slowly it will overheat. After doing this the next day the coolant reservior will be empty. I have not seen any coolant on the ground under the car. I have tried this several times and it is repeatable. What am I missing?