Temperature gauge issue

[Kokomo]

Hi all, I have a 2018 A4 Avant 1.4 petrol, my digital temperature gauge reaches operating temperature within 5 minutes (4 bars on the screen) within 2 minutes it then drops to 1 bar then works its way back up to 4 over about 20 minutes, anyone know what the problem is? Airlock? Thermostat? Many thanks 

[cliffcoggin]

Is that new behaviour or has it always been like that? Does the guage stay at the normal reading on long journeys once it has risen for the second time?