Thanks T:

I bought the thermostat directly from our MB dealer. So I really don't know what temp it runs at. Sorry for calling it a thermometer....work term....

The hill is about 3 KM and runs to an elevation of 1200 ft.

So your theory might be correct and the coolant might be too hot at the bottom of the hill. I guess the engine runs pretty hard getting the beast up the hill and the coolant boils.

The radiator was changed by MB a few years ago, so I assume it must be a good one.

So your suggestion is to change the thermostat to one that runs cooler? I've read about running without a thermostat, what do you think?

It was a VERY difficult project to change the thermostat as one hose on the housing was really difficult to work with.....

How would I find out what temp the thermostat runs at?

Many thanks T! and everyone else....