I had the same problem, cured it with a new thermostat! If you are mildly handy you can replace it yourself. Thermostats usually cost less than $10-$15 (make sure to get the gasket with it). To find the thermostat housing follow the top radiator hose back to the engine. The housing that the hose is connected to is the housing. It's held on with 2 metric bolts, you will probably need a deep socket, I think there is a pin on the top of one bolt. If you move quickly you can replace the thermostat with out drain the radiator some (make sure the new gasket is in place prior to starting). Once the new thermostat is in place and the housing is tight you will need to start the engine and let it warm up. While it's warming up add some antifreeze mix to the overflow container to replace what you lost. Once the engine is warm, excess air needs to be bleed off. To do that open the brass valve on the thermostat housing a bit. Once antifreeze starts coming out close it. I generally wait a couple of minutes and repeat the procedure. To you lurkers who are in the know, did I leave anything out?