Now that it's getting down to freezing, my 240D is running cold: at 55 mph, the temperature gauge hovers around the second mark, the one between 100 and 175 (137.5 deg. F., I'm guessing?). I can't figure out what's going on.

Some info: Warm-up time seems no longer than last year's winter weather warm-ups. The thermostat is closed at start-up (no coolant through top radiator hose); come the second mark on the temperature gauge, the thermostat has opened somewhat (pressure inside top radiator hose). The temperature increases to around 175 deg. F. when I get up to 80 and drops to the second mark when I get back down to 55. The temperature appears to increase slightly during idle at stoplights. All through the summer, the temperature generally read just below 175 deg. F. Last winter, the engine would regularly run just below 175 deg. F., regardless of speed. I replaced the temperature sender last summer.

Any ideas as to what I'm looking at would be appreciated.

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'Betsy the Getaway Car' : 1978 MB 240D, 582k
'The Mistress of the Sea' : 2001 Giant Boulder, 10.30k