Delbourt

One point I forgot to mention and I was only reminded of it as I thumbed through the pages of the pdf from dtafast, and that is the fact that the spro range and probably others including OEM ECUs will have a default value.
Looking at the small changes in output it may be that the default is close to a temperature equivalent of the thermostat opening temperature.
If that is the case then once the car is running and warm it will continue to run and re start until it cools down a lot.
It may also be a case of an aftermarket instrument has been fitted in the past with different characteristics.

I agree that tracking could be cause particularly if the car had been used with road salt present.
Either way my thoughts are check the water transducers and associated wiring first.