This may sound a bit out there but I had one client come in with a car that was 'constantly running hot' turned out the temp sender had lost it's mind.. 25$ later all was well.

Have you had the system pressure tested?

Jonathan

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Blue Ridge Mercedes
Jonathan Hodgman
http://www.blueridgemb.com/
Enthusiast Service, Restoration & Tuning.
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Located in the Atlanta area
Specializing in all pre and post merger AMG's including Hammers and DOHC M117 engines.
Mercedes Repair Atlanta