Maybe your water pump is starting to head south? I've heard that some M-B water pumps use plastic impellers, and blades can break off -- but I don't know what models this may affect. Such a problem would result in less coolant flow and higher temperatures. I'm not sure how to check for that, but I bet someone here on the forum can tell you. If that's it, repair or replace before the whole thing goes and you warp a head or blow a gasket overheating! You're probably fine right up to 120, but keep a close eye on the needle. Good luck...

Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI