I believe that one has the mechanical temp gauge. The accuracy can certainly degrade, but I think the repeatability would tend to stay pretty close, unlike the electrical gauges.

I second the air bubble and sticking thermostat theories. Also, what kind of shape is the fan belt in?

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.