The culprit is the ribbon cable that is connected to the various displays...the conductive strip deteriorates at certain areas, causing pixels to drop out.
This should have been a recall campaign by Mercedes as many of the models equipped with the same displays suffer this problem...but then again, you know Mercedes and how they bend over backwards for this sort of thing.
My CLK has the same ailment...the temperature gauge and the shift indicator/clock. I find that when the car is out on warm days, it seems to restore itself...I guess the conductive lines expand enough to complete the circuit.
The only fix offered by the dealer is a full cluster replacement...$1200 or so!
There is a guy on eBay who will offer to repair your cluster for a few hundred $$$.
There was also a thread posted by a member who apparently has some skill with electronics. He used care, a powerful magnifying lens and a conductive pen to restore the ribbon lines and get his cluster to work properly.