Does your car have a viscous fan clutch? I used to have a 400SEL, but didn't spend any time under the hood, so I don't recall.

If it does, then that could very well be the problem. It would explain why around town you're heating up, and cooling down while on the highway.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles