I've got, what can only be described as a rather unusual water temperature reporting fault (well to me anyway).
What happens is if I start the car a short time after it has got fully up to temperature, say stopping for petrol is a good example (having a 2T that happens quite often), I find that the water temp gauge has crept a little (just over 90 before starting with ignition on 'on' and once started) and after a few seconds of the car running the gauge drops to 70 (as in effectively switches off) and the radiator cooling fans start up.
I assume this happens as a precautionary measure as the ECU doesn't know what the actual water temp is so plays it safe. It doesn't pull too much performance though, maybe drops the redline a little to just over 6k but it still pulls etc its just it becomes very irritating.
If, on the other hand, you don't restart the car until it has cooled a little (say it has dropped to 75-80deg C) before restarting this doesn't happen and the gauge records correctly.
If the fans cut in they will stay on for the duration of the journey until the car is switched off and allowed to cool. Usually when the car is restarted (say the next morning) the car will warm up normally and record the correct temp for the duration of any journey providing the car is not switched off and back on again in a short period while hot.
I say usually as maybe the fans will kick in again (if they on when the car was last used) when cold in the morning but will be OK the next time it is left to cool but this only happens say 1 time in 10 of the fault occurring.
Apologies for the length but the fault is at least mostly consistent so thought it best to give a full description of it