Hi there trueog,

Thermostat seldom give problem. It will cause the temp to rise to the red zone if it's stuck. However the radiator cap is more likely to cause problem if the rubber seal and spring tensioner fail. Anyway get it replace as it's relatively cheap.

Since you mention you need to top-up coolant every 10 days, I bet the radiator is leaking at the end tanks. The radiator is made from aluminium but the end tanks from plastic. Due to time and heat, the plastic can crack and cause coolant to leak. Inspect carefully as it is quite difficult to see. Happened to my 190E last time. The water pump also can leak at the rubber seal.

The trick is to start observing the suspected area the moment you start the car in the mornings from a cold engine. You will soon see coolant slowly seep out from the leak in a few minutes time just as soon as the thermostat opens.

On my case both the radiator end tank and the water pump leaked at the same time.

Hope this helps.

Cheers.