This same thing happened to me after I had the cooling system flushed and filled in my 300D. I beat myself to death to find the problem. I did what you are doing and nothing made sense.
I left it alone for the rest of the year and flushed the cooling system again the next year. Now it is still hotter than original, but less than it was after the first flush. The only thing different was the type of anti-freeze that was used. Now I am using Havoline, but I'm not sure what was used before (probably Prestone for Aluminum).
I doubt this made much of a difference, but you may wish to try a different anti-freeze.
What can it hurt? You've tried everything else
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles