I have 2.0 turbo petrol, A20NHT, had the same problem. It is foulty / stuck termostat. Cost ~ 30 €, only cartridge is replaced.
For gauge, it is normal to stay on 90, if drop engine is being cooled, if up engine is overheated.
If you have digital temperature on DIS it would go from 89 to 100 (fan starts)