Capacitors in relay could need replacement.

The vac flap, Vacuum actuated controller flap can be seen standing in front of the car on the left it controls the amount of air entering the intake manifold. I highly doubt that will be your problem.

What is the mileage on the car and would you happen to have a very gradual loss of coolant?

... (if it needs saying,) assuming you have the valves properly adjusted?

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'83 300 SD
'05 E320 4Matic
'06 BMW 530 xi
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr

131k miles, no coolant loss.

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'92 300D 2.5
'99 E300TD

No valves to adjust this is a 602

Unique problem I would still think it may be the wrong resistance sensor. I would search for a temp sensor thread started by member BillGrissom who lists a chart which I believe I confirmed at some point when, I was having similar problems. It's the chart which goes from about 20 ohms to around 400 ohms. Can you check the resistance of both at ambient temp to see the difference if you wish I'll. do the same as I know mine is good.