Erratic coolant temperature gauge

[Alluminio]

After a few trouble free months, my first niggle.

I noticed this first a couple of weeks ago when it fist happened. It now appears to be happening with more regularity.

Occasionally when starting the car from cold, the coolant temperature gauge needle progresses as normal and then shortly after reaching 90 degrees, it either slowly drops to the lower mark or drops quickly.

Stopping the car will see it return to normal operation, or it will move to the 90 degrees mark after driving for a short while.

Yesterday evening the temperature indicator rose to 90 degrees then quickly dropped to the lower nark and did not move up again.

The car is not overheating and the interior heat appears to be OK.

Temperature sensor or thermostat?

If the former, whereabouts is it located.

Any advice much appreciated.

TIA

Mark