When you replace the thermostat, don't overlook an important but tiny detail -- important if your thermostat's bottom housing lies at an angle (i.e., is not horizontal).
On the MB thermostats, there is a little "ball" valve (or, on some models just a hole) -- so when you install the thermostat, it should be rotated and positioned so that this valve is at the highest point. The purpose is to help to quickly purge air out of the system.