Yes, they do that. I bet your temperature changes when you turn on the lights, press the brakes, the cooling fan comes on? Basically anything electrical. There’s a fault in the earth return path from the dash to the chassis which causes it. Have heard of a couple of people move the dash earth direct to the chassis, but never really heard if it fixed the problem.
The blue sender (-> dash/stack) is also a “single wire” type which is affected by electrical load. In most cases, a 1°C rise in temperature
can be seen when turning on each item of electrical equipment, e.g. headlights, heater fan, windscreen wipers.
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So your temperature rises far too much when jumping up to 10° on an added electrical load. I think you should check and clean all earthing connections. Especially the earthing wire between the engine and the chassis. It seems that there is an higher than usual ohm resistance at least at one of the earth connections. Bad earth connections can cause a lot of other bad things. The loading current of the alternator can even ‘eat’ a corrosion hole into your cooling system when a part of the current finds it’s way via the electrolytic cooling fluid to a ‘better’ earthing point (radiator, tubes, heater, etc.).